[x86] improve AVX lowering of vector zext

If we know the 2 halves of an oversized zext-in-reg are the same,
don't create those halves independently.

I tried several different approaches to fold this, but it's difficult
to get right during legalization. In the default path, we are creating
a generic shuffle that looks like an unpack high, but it can get
transformed into a different mask (a blend), so it's not
straightforward to match that. If we try to fold after it actually
becomes an X86ISD::UNPCKH node, we can't be sure what the operand node
is - it might be a generic shuffle, or it could be some x86-specific op.

From the test output, we should be doing something like this for SSE4.1
as well, but I'd rather leave that as a follow-up since it involves
changing lowering actions.

Differential Revision: https://reviews.llvm.org/D59777

llvm-svn: 357129