[Assignment Tracking][NFC] Avoid doing some work when maps have same keys

Where the new checks have been added, `SymmetricDifference` - still being built
- contains entries for variables present in `A` and not in `B`.  If
`SymmetricDifference` is empty at this point it means the variables (map keys)
in `A` are a subset of those in `B`, so if `A` and `B` are the same size then
we know they're identical.

This reduces the number of instructions retired building some of the CTMark
projects in a ReleaseLTO-g configuration (geomean change -0.05% with the best
improvement being -0.24% for tramp3d-v4)

Reviewed By: StephenTozer

Differential Revision: https://reviews.llvm.org/D144621