[globalisel] Support trivial COPY in GISelKnownBits

Summary: Allow GISelKnownBits to look through the trivial case of TargetOpcode::COPY

Reviewers: aditya_nandakumar

Subscribers: rovka, hiraditya, volkan, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67131

llvm-svn: 370955