[InstCombine] visitTrunc - pass through undefs for trunc(shift(trunc/ext(x),c)) patterns

Based on the recent patches D88475 and D88429 where we are losing undef values due to extension/comparisons.

I've added a Constant::mergeUndefsWith method that merges the undef scalar/elements from another Constant into a specific Constant.

Differential Revision: https://reviews.llvm.org/D88687