[InstCombine] generalize fold for mask-with-signbit-splat

(iN X s>> (N-1)) & Y --> (X < 0) ? Y : 0

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

I was looking at a missing abs() transform and found my way to this
generalization of an existing fold that was added with D67799.
As discussed in that review, we want to make sure codegen handles
this difference well, and for all of the targets/types that I
spot-checked, it looks good.

I am leaving the existing fold in place in this commit because
it covers a potentially missing icmp fold, but I plan to remove
that as a follow-up commit as suggested during review.

Differential Revision: https://reviews.llvm.org/D111410