[NFC][regalloc] Make VirtRegAuxInfo part of allocator state

All the state of VRAI is allocator-wide, so we can avoid creating it
every time we need it. In addition, the normalization function is
allocator-specific. In a next change, we can simplify that design in
favor of just having it as a virtual member.

Differential Revision: https://reviews.llvm.org/D88499