[Analysis] add query to get splat value from array of ints

I was debug stepping through an x86 shuffle lowering and
noticed we were doing an N^2 search for splat index. I
didn't find the equivalent functionality anywhere else in
LLVM, so here's a helper that takes an array of int and
returns a splatted index while ignoring undefs (any
negative value).

This might also be used inside existing
ShuffleVectorInst/ShuffleVectorSDNode functions and/or
help with D72467.

Differential Revision: https://reviews.llvm.org/D74064