Copy utilities updated and added for MI flags

Summary: This patch adds a GlobalIsel copy utility into MI for flags and updates the instruction emitter for the SDAG path. Some tests show new behavior and I added one for GlobalIsel which mirrors an SDAG test for handling nsw/nuw.

Reviewers: spatel, wristow, arsenm

Reviewed By: arsenm

Subscribers: wdng

Differential Revision: https://reviews.llvm.org/D52006

llvm-svn: 342576