[LICM][NFC] Restructure pointer invalidation API in terms of MemoryLocation

Main value is just simplifying code.  I'll further simply the argument handling case in a bit, but that involved a slightly orthogonal change so I went with the mildy ugly intermediate for this patch.

Note that the isSized check in the old LICM code was not carried across.  It turns out that check was dead.  a) no test exercised it, and b) langref and verifier had been updated to disallow unsized types used in loads.

llvm-svn: 339930