[CallSite removal][MemCpyOptimizer] Replace CallSite with CallBase. NFC

There are also some adjustments to use MaybeAlign in here due
to CallBase::getParamAlignment() being deprecated. It would
be a little cleaner if getOrEnforceKnownAlignment was migrated
to Align/MaybeAlign.

Differential Revision: https://reviews.llvm.org/D78345