Change allowsUnalignedMemoryAccesses to take type argument since some targets

Change allowsUnalignedMemoryAccesses to take type argument since some targets
support unaligned mem access only for certain types. (Should it be size
instead?)

ARM v7 supports unaligned access for i16 and i32, some v6 variants support it
as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79127 91177308-0d34-0410-b5e6-96231b3b80d8