[InstCombine] canonicalize bitcast after insertelement into undef

[InstCombine] canonicalize bitcast after insertelement into undef

We have a transform in the opposite direction only for the x86 MMX type,
Other types are not handled either way before this patch.

The motivating case from PR45748:
https://bugs.llvm.org/show_bug.cgi?id=45748
...is the last test diff. In that example, we are triggering an existing
bitcast transform, so we reduce the number of casts, and that should give
us the ideal x86 codegen.

Differential Revision: https://reviews.llvm.org/D79171