[MemorySSA] Always perform MemoryUses liveOnEntry optimization on MSSA construction

Fixes invariant memory regressions in future DSE patches.

Also add a flag to print<memoryssa> to not ensure optimized uses to test this.

Noticeable compile time regression [1], but a future DSE change that depends on this more than makes up for it.

[1] https://llvm-compile-time-tracker.com/compare.php?from=9d5466849a770eeab222d5a5890376d3596e8ad6&to=95682dbe11d76a3342870437377216e96b167504&stat=instructions:u

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D152859