[MIPS GlobalISel] Skip copies in addUseDef and addDefUses

Skip copies between virtual registers during search for UseDefs
and DefUses.
Since each operand has one def search for UseDefs is straightforward.
But since operand can have many uses, we have to check all uses of
each copy we traverse during search for DefUses.

Differential Revision: https://reviews.llvm.org/D64486

llvm-svn: 365744