[MemCpyOpt] Use LocationSize instead of ints; NFC
Trying to keep these patches super small so they're easily post-commit
verifiable, as requested in D44748.
srcSize is derived from the size of an alloca, and we quit out if the
size of that is > the size of the thing we're copying to. Hence, we
should always copy everything over, so these sizes are precise.
Don't make srcSize itself a LocationSize, since optionality isn't
helpful, and we do some comparisons against other sizes elsewhere in