[InsttCombine] fold insertelement of constant into shuffle with constant operand (PR29126)

The motivating case occurs with SSE/AVX scalar intrinsics, so this is a first step towards
shrinking that to a single shufflevector.

Note that the transform is intentionally limited to shuffles that are equivalent to vector
selects to avoid creating arbitrary shuffle masks that may not lower well.

This should solve PR29126:
https://llvm.org/bugs/show_bug.cgi?id=29126

Differential Revision: https://reviews.llvm.org/D23886

llvm-svn: 280504