[MLGO] Add per-instruction MBB frequencies to regalloc dev features

This commit adds in two new features to the ML regalloc eviction
analysis that can be used in ML models, a vector of MBB frequencies and
a vector of indicies mapping instructions to their corresponding basic
blocks. This will allow for further experimentation with per-instruction
features and give a lot more flexibility for future experimentation over
how we're extracting MBB frequency data currently.

Reviewed By: mtrofin, jacobhegna

Differential Revision: https://reviews.llvm.org/D134166