[llvm] VPlan/PatternMatch: introduce m_c_Mul (NFC) (PR #93950)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

This codepath is quite well-tested:

```
  LLVM :: Transforms/LoopVectorize/ARM/mve-gather-scatter-tailpred.ll
  LLVM :: Transforms/LoopVectorize/RISCV/strided-accesses.ll
  LLVM :: Transforms/LoopVectorize/runtime-check-needed-but-empty.ll
  LLVM :: Transforms/LoopVectorize/runtime-checks-hoist.ll
  LLVM :: Transforms/LoopVectorize/select-reduction.ll
  LLVM :: Transforms/LoopVectorize/version-stride-with-integer-casts.ll
```

https://github.com/llvm/llvm-project/pull/93950