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

[Antonio Frighetto via llvm-commits]

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antoniofrighetto wrote:

Perhaps we could include `!extract oneuse(op.with.overflow),1` in the pattern, have something like (and adjust the limit):
```cpp
WithOverflowInst *UnusedWO;
if (match(&I, m_CombineOr(m_ExtractValue<1>(m_OneUse(m_WithOverflowInst(UnusedWO))),
                          m_Not(m_ExtractValue<1>(m_OneUse(m_WithOverflowInst(UnusedWO)))))))
  Found = true;
```
But TBH, I believe the current code should be fine (regardless of the test).

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