[MemorySSA] Remove -enable-mssa-loop-dependency option

This option has been enabled by default for quite a while now.
The practical impact of removing the option is that MSSA use
cannot be disabled in default pipelines (both LPM and NPM) and
in manual LPM invocations. NPM can still choose to enable/disable
MSSA using loop vs loop-mssa.

The next step will be to require MSSA for LICM and drop the
AST-based implementation entirely.

Differential Revision: https://reviews.llvm.org/D108075