[llvm] [X86][CodeGen] Fix crash when commute operands of Instruction for code size (PR #79245)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> Difficult for me to create a test for this, maybe @gulfemsavrun could provide a reproducer later.

```
$ cat tmp.ll
define <4 x float> @foo(<4 x float> %a, <4 x float> %b) optsize {
  %r = shufflevector <4 x float> %b, <4 x float> %a, <4 x i32> <i32 0, i32 5, i32 2, i32 3>
  ret <4 x float> %r
}

$ llc < tmp.ll -mattr=+avx2
        .text
        .file   "<stdin>"
Unreachable!
UNREACHABLE executed at /export/users/pengfeiw/llvm-project/llvm/lib/Target/X86/X86InstrInfo.cpp:2396!
```

https://github.com/llvm/llvm-project/pull/79245