[SCEV] Improve zext(A /u B) and zext(A % B)

Summary:
Try to match udiv and urem patterns, and sink zext down to the leaves.

I'm not entirely sure why some unrelated tests change, but the added <nsw>s seem right.

Reviewers: sanjoy

Subscribers: jlebar, hiraditya, bixia, llvm-commits

Differential Revision: https://reviews.llvm.org/D48338

llvm-svn: 335197