[InstCombine] cttz(zext(x)) -> zext(cttz(x)) if the 'ZeroIsUndef' parameter is 'true' (PR50172)

Zext doesn't change the number of trailing zeros, so narrow cttz(zext(x)) -> zext(cttz(x)) if the 'ZeroIsUndef' parameter is 'true'.

Proofs:
https://alive2.llvm.org/ce/z/o2dnjY

Solves https://bugs.llvm.org/show_bug.cgi?id=50172

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D101582