Resolve a long-standing FIXME in memcpyopt.

Inspecting the downstream use of the cpyAlign, it is clear that
`performCallSlotOptzn` is expecting it to represent the alignment
of the copy destination, not the minimum of the src and dest
alignments. This patch renames the parameter to make this more
obvious.

I believe this change is NFC, because the downstream code has
alignment checks such that it all works out in the end. I have not
been able to construct a test case that actually triggers a change
in output.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D140603