[InstCombine] Canonicalize insert splat sequences into an insert + shuffle

[InstCombine] Canonicalize insert splat sequences into an insert + shuffle

This adds a combine that canonicalizes a chain of inserts which broadcasts
a value into a single insert + a splat shufflevector.

This fixes PR31286.

Differential Revision: https://reviews.llvm.org/D27992

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@290641 91177308-0d34-0410-b5e6-96231b3b80d8