[X86] Add a DAG combine pre type legalization to widen division by constant...

[X86] Add a DAG combine pre type legalization to widen division by constant splat on narrow vectors to avoid scalarization

This is another patch for -x86-experimental-vector-widening. This pre widens narrow division by constants so that we can get pass the legal type check in the generic DAG combiner. Otherwise we end up scalarizing.

I've restricted this to splats for now because it was easy to just call DAG.getConstant. Not sure what we should do for non-splat? Increase the element size?Widen the constant vector by padding with 1?

Differential Revision: https://reviews.llvm.org/D54919

llvm-svn: 347898