[MemCpyOpt] Add test scaffolding for MSSA based MemCpyOpt

This adds an -enable-memcpyopt-memoryssa option that currently does
nothing apart from requiring MSSA as a dependency. The tests are
split to run both with the option disabled and enabled. I went with
this rather than the separate directory DSE uses, as I found it
convenient to have a direct side-by-side comparison of differences.

Differential Revision: https://reviews.llvm.org/D89206