Reapply "AMDGPU: Scavenge register instead of findUnusedReg"

This reapplies r356149, using the correct overload of findUnusedReg
which passes the current iterator.

This worked most of the time, because the scavenger iterator was moved
at the end of the frame index loop in PEI. This would fail if the
spill was the first instruction. This was further hidden by the fact
that the scavenger wasn't passed in for normal frame index
elimination.

llvm-svn: 357098