[InstCombine] fold freeze of partial undef/poison vector constants

We can always replace the undef elements in a vector constant
with regular constants to get rid of the freeze:
https://alive2.llvm.org/ce/z/nfRb4F

The select diffs show that we might do better by adjusting the
logic for a frozen select condition. We may also want to refine
the vector constant replacement to consider forming a splat.

Differential Revision: https://reviews.llvm.org/D123962