[llvm] [X86][GlobalIsel] support G_FABS for f80 (PR #136718)

[via llvm-commits]

mahesh-attarde wrote:

I am working on i686 test. There seems to be something off. I am collecting information on it.

```
 bb.0 (%ir-block.0):
  renamable $eax = MOV32ri 2147483647
  renamable $eax = AND32rm killed renamable $eax(tied-def 0), %fixed-stack.0, 1, $noreg, 0, $noreg, implicit-def dead $eflags :: (invariant load (s32) from %fixed-stack.0)
  $fp0 = COPY killed renamable $eax
  RET 0, implicit killed $fp0
```
This code results, This is part of lowerReturn in GISEL. I suspect fp0 should not even be created. Let me know if I am wrong.
```
llc: /llvm-project/llvm/lib/Target/X86/X86FloatingPoint.cpp:317: unsigned int getFPReg(const llvm::MachineOperand&): Assertion `Reg >= X86::FP0 && Reg <= X86::FP6 && "Expected FP register!"' failed. 
```

https://github.com/llvm/llvm-project/pull/136718