We already handle blendv(x,y,bitcast(sext(m))) -> select(m,x,y) cases, but this adds support for peeking through one-use shuffles as well. VectorCombine should already have canonicalized the IR to shuffle(bitcast(...)) for us.
The particular use case is where we have split generic 256/512-bit code to use target-specific blendv intrinsics (e.g. AVX1 spoofing AVX2 256-bit ops).
Fixes#58895
7de7f50fc9