Some application code operating on generic pointers (that then gete
initialized to buffer fat pointers) may perform tests against nullptr.
After address space inference, this results in comparisons against
`addrspacecast (ptr null to ptr addrspace(7))`, which were crashing.

However, while general casts to ptr addrspace(7) from generic pointers
aren't supposted, it is possible to cast null pointers to the all-zerose
bufer resource and 0 offset, which this patch adds.

It also adds a TODO for casting _out_ of buffer resources, which isn't
implemented here but could be.

Since e39f6c1844fab59c638d8059a6cf139adb42279a opt will infer the
correct datalayout when given a triple. Avoid explicitly specifying it
in tests that depend on the AMDGPU target being present to avoid the
string becoming out of sync with the TargetInfo value.
Only tests with REQUIRES: amdgpu-registered-target or a local lit.cfg
were updated to ensure that tests for non-target-specific passes that
happen to use the AMDGPU layout still pass when building with a limited
set of targets.

Reviewed By: shiltian, arsenm

Pull Request: https://github.com/llvm/llvm-project/pull/137921

This PR updates AMDGPULowerBufferFatPointers to use the
InstSimplifyFolder
when creating IR during buffer fat pointer lowering.

This shouldn't cause any large functional changes and might improve the
quality of the generated code.

Attempting to pass a `ptr addrspace(7)` to functions that take `ptr`
arguments produces undesirable `addrspacecast(addrspacecast(p8 x to p7)
to p0) => addrspacecast(p8 x to p0)` folds. This results in illegal GEP
operations on buffer resources, which can't be GEP'd. (However, note
that, while unimplemneted, addressspacecast from ptr addrspace(7) to ptr
is legal - it's just an effective address computation)

To resolve this problem, and thus prevent illegal
`getelementptr T, ptr addrspace(8) %x, ...` s from being produces, this
commit extends amdgcn.make.buffer.rsrc to also be variadic in its result
type, auto-upgrading old manglings.

The logic for handling a make.buffer.rsrc in instruction selection
remains untouched and expects the output type to be a ptr addrspace(8),
as does the Clang lowering for its builtin (the pointer-to-pointer
version might want a different name in clang). LowerBufferFatPointers
has been updated to lower
amdgcn.make.buffer.rsrc.p7.p* to amdgcn.make.buffer.rsrc.p8.p* .

This'll also make exposing buffer fat pointers in Clang easier, since
you don't have to cast between a `__amdgcn_rsrc_t` and a pointer.

The lowering for GEP didn't properly support the case where the pointer
argument was being implicitly broadcast by a vector of indices. Fix
that.

---------

Co-authored-by: Matt Arsenault <arsenm2@gmail.com>

This commit usis the `nuw` flag on `getelemnetptr` to set the `nuw` flag
on buffer offset additions, and also moves from `inbounds` to the looser
`nusw` for the existing case.

For ptrtoint that truncates to the offset only, the expansion generated
a shift by the bit width, which is poison. Instead, we should return the
offset directly.

(The same problem exists for the constant expression case, but I plan to
address that separately, and more comprehensively.)

OffAccum will never be nullptr now, instead check for a zero
constant.

Use emitGEPOffset() to emit the GEP offset, which already has all the
necessary logic.

This also fixes the nuw flag incorrectly being set on the offset
calculation, while only nsw is implied by inbounds.