[x86] X86ISelLowering zext(add_nuw(x, C)) --> add(zext(x), C_zext)

Currently X86ISelLowering has a similar transformation for sexts:
sext(add_nsw(x, C)) --> add(sext(x), C_sext)

In this change I extend this code to handle zexts as well.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D23359

llvm-svn: 278520