Use separate memcpy source/dest alignments in LLVM 7

[hanna-kruppe]

LLVM trunk recently switched from a single alignment argument that applies to source and destination to annotating both the source and destination pointers with their respective alignment. See https://lists.llvm.org/pipermail/llvm-dev/2018-March/122143.html and the rest of that thread for details.

After we upgrade to LLVM 7, we should try to make use of that anywhere we know that the source and destination have different alignments. cc @eddyb