First, expandFMINIMUM_FMAXIMUM should be a never-fail API. The client
wanted it expanded, and it can always be expanded. This logic was tied
up with what the VectorLegalizer wanted.
    
Prefer using the min/max opcodes, and unrolling if we don't have a
vselect.
This seems to produce better code in all the changed tests.

These would fail to select.

These were using patterns copied from older tests, before non-kernel
functions were supported and manually written checks. Also stop using
-flat-for-global, which only exists to try to share tests between SI/CI
and VI+.

This was also missing test coverage, we're incorrectly forming
maximum3/minimum3 pre-gfx12. This is a pre-commit before fixing that.

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
```

Co-authored-by: Stanislav Mekhanoshin <Stanislav.Mekhanoshin@amd.com>