[InstCombine] add casts from splat-a-bit pattern if necessary

Splatting a bit of constant-index across a value:
sext (ashr (trunc iN X to iM), M-1) to iN --> ashr (shl X, N-M), N-1
If the dest type is different, use a cast (adjust use check).

https://alive2.llvm.org/ce/z/acAan3

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D124590