[AMDGPU] Add implicit uses to AGPR copy MIR tests

Some tests were using liveins or IMPLICIT_DEFs to add fake live
registers, but that only works if you track liveness forwards. Add some
implicit uses too, so that it also works if you track liveness
backwards.

Some of these tests were using the regmask amdgpu_allvgprs but that is a
clobber not a use.

Differential Revision: https://reviews.llvm.org/D150570