[CodeGen] Use RegUnits in RegisterClassInfo::getLastCalleeSavedAlias (#79996)

Change the implementation of getLastCalleeSavedAlias to use RegUnits
instead of register aliases. This is much faster on targets like AMDGPU
which define a very large number of overlapping register tuples.

No functional change intended. If PhysReg overlaps multiple CSRs then
getLastCalleeSavedAlias(PhysReg) could conceivably return a different
arbitrary one, but currently it is only used for some debug printing
anyway.

Differential Revision: https://reviews.llvm.org/D146734