[MemoryLocation] Use LocationSize instead of ints; NFC
Trying to keep these patches super small so they're easily post-commit
verifiable, as requested in D44748.
This one sadly isn't *super* small, but all of the changes here are
- libfuncs that are passed a constant size (memcpy, memset, ...)
- instructions that store/load a constant size
So they have to be precise