Check if register class was changed in constrainOperandRegClass()

NFC
When no actual change happens there's no need to notify the
observers about the fact the register class is being constrained.
So we should avoid notifying observers when no change has
happened, because this can dramatically affect compile
time for particular test cases.

Reviewed By: dsanders, arsenm

Differential Revision: https://reviews.llvm.org/D122615