Previously we called ignoreCSRForAllocationOrder on every alias of every
CSR which was expensive on targets like AMDGPU which define a very large
number of overlapping register tuples.
On such targets it is simpler and faster to call
ignoreCSRForAllocationOrder once for every physical register.
Given the only spot where ::ignoreCSRForAllocationOrder is used is guarded by CalleeSavedAliases and that one is populated with regunits (after your other change), I think we should go through the regunits too here.