AMDGPU/GlobalISel: Do not use terminator copy before waterfall loops

AMDGPU/GlobalISel: Do not use terminator copy before waterfall loops

Stop using the _term variants of the mov to save the initial exec
value before the waterfall loop. This cannot be glued to the bottom of
the block because we may need to spill the result register. Just use a
regular mov, like the loops produced on the DAG path. Fixes some
verification errors with regalloc fast.