use TLI.allowsMemoryAccess() to check if memory accesses are fast; NFCI

This consolidates use of isUnalignedMem32Slow() in one place.
There is a slight change in logic although I'm not sure that it would ever
come up in the real world: we were assuming that an alignment of the type 
size is always fast; now, we actually check the data layout to confirm that.

llvm-svn: 245382