[TargetLowering] Make allowsMemoryAccess methode virtual.

Rename old function to explicitly show that it cares only about alignment.
The new allowsMemoryAccess call the function related to alignment by default
and can be overridden by target to inform whether the memory access is legal or
not.

Differential Revision: https://reviews.llvm.org/D67121

llvm-svn: 372935