[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
either to:

• libfuncs that are passed a constant size (memcpy, memset, ...)
• instructions that store/load a constant size

So they have to be precise

llvm-svn: 350017