[InstCombine] Make folding (X >s -1) ? C1 : C2 --> ((X >>s 31) & (C2 - C1)) +...

[InstCombine] Make folding (X >s -1) ? C1 : C2 --> ((X >>s 31) & (C2 - C1)) + C1 support splat vectors

This also uses decomposeBitTestICmp to decode the compare.

Differential Revision: https://reviews.llvm.org/D36781

llvm-svn: 311044