[llvm] VPlan: simplify mul with zero (PR #127521)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> Does a mul of zero ever get introduced by the loop vectorizer? Otherwise I'd assume InstCombine would remove it from the input beforehand

See surrounding code. Yes, LV introduces plenty of trivial constructs like mul with 1, not(not(V)) etc. when building the VPlan.

https://github.com/llvm/llvm-project/pull/127521