[LoopSink] Use MemorySSA with legacy pass manager

LoopSink with the legacy pass manager still uses AST, because we
can't compute MemorySSA conditionally. I think now that the legacy
pass manager will be removed soon(TM) we don't need to care about
compile-time impact here anymore. Additionally, since MemorySSA is
no longer eagerly optimized, the impact is actually not that high
anymore (~0.2% geomean regression on CTMark).

This just makes legacy PM and new PM behavior line up -- as a
followup I'll drop these options entirely and make MemorySSA use
mandatory.

Differential Revision: https://reviews.llvm.org/D123216