[InstrProf][NFC] Save profile bias to function map

Add a map from functions to load instructions that compute the profile bias. Previously we assumed that if the first instruction in the function was a load instruction, then it must be computing the bias. This was likely to work out because functions usually start with the `llvm.instrprof.increment` instruction, but optimizations could change this. For example, inlining into a non-profiled function.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D114319