[InstCombine] Zero-extend shift amounts in narrow funnel shift ops

An issue arose when handling shift amounts while performing
narrowed funnel shifts simplification. Specifically, shift
amounts were incorrectly truncated when their type was
narrower than the target bit width. This has been addressed
by zero-extending `ShAmt` in such cases.

Fixes: https://github.com/llvm/llvm-project/issues/71463.

Proof: https://alive2.llvm.org/ce/z/5draKz.