Revert rG25a97c3a43d7 - "[InstCombine] visitCallInst - retain undefs in vector...

Revert rG25a97c3a43d7 - "[InstCombine] visitCallInst - retain undefs in vector funnel shift amounts"

This reverts commit 25a97c3a.

We have other constant folds that fold undef funnel shift amounts to 0 - so we need to be consistent.

If we end up with regressions where we lose a splat shift amount pattern we'll have to investigate other canonicalizations, but matchFunnelShift currently protects us from that.