Specialize matchWidth for Literals

This avoids generating extra nodes to extend/contract constants
and instead just generates new constants.
For inline Mems, implement masked writes as RMW operations

This is simpler and sometimes better.  Specialized Mem backends should
still support masked write ports natively where beneficial.
Eliminate some useless node assignments due to toBits calls
Don't randomize C++ Mem reads when performing masked writes

The implementation of masked writes relies on this.