[X86] Match bitcast of vxi1 to pmovmsk

Summary:
Add an early combine to match patterns such as:
  (i16 bitcast (v16i1 x))
  ->
  (i16 movmsk (v16i8 sext (v16i1 x)))

This combine needs to happen early enough before
type-legalization scalarizes the result of the setcc.

Reviewers: igorb, craig.topper, RKSimon

Subscribers: delena, llvm-commits

Differential Revision: https://reviews.llvm.org/D33311

llvm-svn: 304406