[llvm] [SimplifyCFG] Treat umul + extract pattern as cheap single instruction. (PR #124933)

[Gábor Spaits via llvm-commits]

spaits wrote:

Okay I found out, that when running this command:
```sh
bin/opt -passes=instcombine,simplifycfg -simplifycfg-require-and-preserve-domtree=1 -S testfile.ll
```
The input BasicBlock (`ThenBB`) for spewill look like this:
```llvm
bb2:                                              ; preds = %bb
  %mul = call { i64, i1 } @llvm.umul.with.overflow.i64(i64 %arg, i64 %arg1)
  %mul.ov = extractvalue { i64, i1 } %mul, 1
  %0 = xor i1 %mul.ov, true
  br label %bb5

  %0 = xor i1 %mul.ov, true

  %mul.ov = extractvalue { i64, i1 } %mul, 1
```

That `xor` is added into the basic block by  `InstCombinePass`.


https://github.com/llvm/llvm-project/pull/124933