Many tests use store to undef as a placeholder use, so just replace
all of these with poison.

This reverts commit ca33649abe5fad93c57afef54e43ed9b3249cd86.

This reverts commit e215a1e27d84adad2635a52393621eb4fa439dc9 as it broke both
hip and openmp buildbots.

This reverts commit adaff46d087799072438dd744b038e6fd50a2d78.

Drop the -O3 checks from default-attributes.hip. I don't know why they
are different on some bots but reverting this is far too disruptive.

This reverts commits 677cc15e0ff2e0e6aa30538eb187990a6a8f53c0 and
78bc1b64a6dc3fb6191355a5e1b502be8b3668e7.

The test CodeGenHIP/default-attributes.hip is failing on multiple bots
even after the attempted fix including the following:
- https://lab.llvm.org/buildbot/#/builders/3/builds/1473
- https://lab.llvm.org/buildbot/#/builders/65/builds/1380
- https://lab.llvm.org/buildbot/#/builders/161/builds/595
- https://lab.llvm.org/buildbot/#/builders/154/builds/1372
- https://lab.llvm.org/buildbot/#/builders/133/builds/1547
- https://lab.llvm.org/buildbot/#/builders/81/builds/755
- https://lab.llvm.org/buildbot/#/builders/40/builds/570
- https://lab.llvm.org/buildbot/#/builders/13/builds/748
- https://lab.llvm.org/buildbot/#/builders/12/builds/1845
- https://lab.llvm.org/buildbot/#/builders/11/builds/1695
- https://lab.llvm.org/buildbot/#/builders/190/builds/1829
- https://lab.llvm.org/buildbot/#/builders/193/builds/962
- https://lab.llvm.org/buildbot/#/builders/23/builds/991
- https://lab.llvm.org/buildbot/#/builders/144/builds/2256
- https://lab.llvm.org/buildbot/#/builders/46/builds/1614

These bots have been broken for a day, so reverting to get everything
back to green.

Removing it from the codegen pipeline induces a lot of test churn
because llc is no longer optimizing out implicit arguments to kernels.

Mostly mechanical, but there are some creative test updates. I preferred
to take the changes as-is in tests where the ABI isn't relevant. In
cases where it's more relevant, or the optimize out logic was too
ingrained in the test, I pre-run the optimization. Some cases manually
add attributes to disable inputs.

Similar to 806761a7629df268c8aed49657aeccffa6bca449.

For IR files without a target triple, -mtriple= specifies the full
target triple while -march= merely sets the architecture part of the
default target triple, leaving a target triple which may not make sense,
e.g. amdgpu-apple-darwin.

Therefore, -march= is error-prone and not recommended for tests without
a target triple. The issue has been benign as we recognize
$unknown-apple-darwin as ELF instead of rejecting it outrightly.

This patch changes AMDGPU tests to not rely on the default
OS/environment components. Tests that need fixes are not changed:

```
  LLVM :: CodeGen/AMDGPU/fabs.f64.ll
  LLVM :: CodeGen/AMDGPU/fabs.ll
  LLVM :: CodeGen/AMDGPU/floor.ll
  LLVM :: CodeGen/AMDGPU/fneg-fabs.f64.ll
  LLVM :: CodeGen/AMDGPU/fneg-fabs.ll
  LLVM :: CodeGen/AMDGPU/r600-infinite-loop-bug-while-reorganizing-vector.ll
  LLVM :: CodeGen/AMDGPU/schedule-if-2.ll
```

The highlight change is a new denormal safe 1ulp lowering which uses
rcp after using frexp to perform input scaling. This saves 2
instructions compared to other implementations which performed an
explicit denormal range change. This improves the OpenCL default, and
requires a flag for HIP. I don't believe there's any flag wired up for
OpenMP to emit the necessary fpmath metadata.

This provides several improvements and changes that were hard to
separate without regressing one case or another. Disturbingly the
OpenCL conformance test seems to have the reciprocal test commented
out. I locally hacked it back in to test this.

Starts introducing f32 rsq intrinsics in AMDGPUCodeGenPrepare. Like
the rcp case, we could do this in codegen if !fpmath were preserved
(although we would lose some computeKnownFPClass tricks). Start
requiring contract flags to form rsq. The rsq fusion actually improves
the result from ~2ulp to ~1ulp. We have some older fusion in codegen
which only keys off unsafe math which should be refined.

Expand rsq patterns by checking for denormal inputs and pre/post
multiplying like the current library code does. We also take advantage
of computeKnownFPClass to avoid the scaling when we can statically
prove the input cannot be a denormal. We could do the same for the rcp
case, but unlike rsq a large input can underflow to denormal. We need
additional upper bound exponent checks on the input in order to do the
same for rcp.

This rsq handling also now starts handling the negated case. We
introduce rsq with an fneg. In the case the fneg doesn't fold into its
user, it's a neutral change but provides improvement if it is foldable
as a source modifier.

Also starts respecting the arcp attribute properly, and more strictly
interprets afn. We were previously interpreting afn as implying you
could do the reciprocal expansion of an fdiv. The codegen handling of
these also needs to be revisited.

This also effectively introduces the optimization
combineRepeatedFPDivisors enables, just done in the IR instead (and
only for f32).

This is almost across the board better. The one minor regression is
for gfx6/buggy frexp case where for multiple reciprocals, we could
previously reuse rematerialized constants per instance (it's neutral
for a single rcp).

The fdiv.fast and sqrt handling need to be revisited next.

https://reviews.llvm.org/D155593