[X86] combineX86ShuffleChain(): canonicalize mask elts picking from splats

Given a shuffle mask, if it is picking from an input that is splat
given the current granularity of the shuffle, then adjust the mask
to pick from the same lane of the input as the mask element is in.
This may result in a shuffle being simplified into a blend.

I believe this is correct given that the splat detection matches the one
just above the new code,

My basic thought is that we might be able to get less regressions
by handling multiple insertions of the same value into a vector
if we form broadcasts+blend here, as opposed to D105390,
but i have not really thought this through,
and did not try implementing it yet.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D107009