[CodeGen] Don't include aliases in RegisterClassInfo::IgnoreCSRForAllocOrder (#80015)

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.

Differential Revision: https://reviews.llvm.org/D146735