[X86] Combine vector anyext + and into a vector zext

Vector zext tends to get legalized into a vector anyext, represented as a vector shuffle with an undef vector + a bitcast, that gets ANDed with a mask that zeroes the undef elements.
Combine this into an explicit shuffle with a zero vector instead. This allows shuffle lowering to match it as a zext, instead of matching it as an anyext and emitting an explicit AND.
This combine only covers a subset of the cases, but it's a start.

Differential Revision: http://reviews.llvm.org/D7666

llvm-svn: 229480