RegisterCoalescer: Enable terminal rule by default for AMDGPU (#161621)
Introduce a target hook to incrementally flip the behavior of
targets with test changes, and start by implementing it for AMDGPU.
This appears to be forgotten switch flip from 2015. This
seems to do a nicer job with subregister copies. Most of the
test changes are improvements or neutral, not that many are
light regressions. The worst AMDGPU regressions are for true16
in the atomic tests, but I think that's due to existing true16
issues.