[X86] Remove dead code from LowerVectorBroadcast

Summary: Splat vectors are canonicalized to BUILD_VECTOR's so the code can be simplified. NFC-ish.

Reviewers: craig.topper, delena, RKSimon, andreadb

Subscribers: RKSimon, llvm-commits

Differential Revision: https://reviews.llvm.org/D26678

llvm-svn: 287643