[InstCombine] Update InstCombine to use poison instead of undef for...

[InstCombine] Update InstCombine to use poison instead of undef for shufflevector's placeholder (1/3)

This patch is for fixing potential shufflevector-related bugs like D93818.
As D93818, this patch change shufflevector's default placeholder to poison.
To reduce risk, it was divided into several patches, and this patch is for InstCombineCasts.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D110226