[X86] Make sure we still emit zext for GR32 to GR64 when the source of the zext is AssertZext

The AssertZext we might see in this case is only giving information about the lower 32 bits. It isn't providing information about the upper 32 bits. So we should emit a zext.

This fixes PR28540.

Differential Revision: https://reviews.llvm.org/D37729

llvm-svn: 313563