[RISCV] Add SchedRead for Merge operands on MASK Pseudos

For Pseudos that end in _MASK or should be forceMasked,
which means that in the case that the instruction should be
mask undisturbed the destination register must be read to
determine the undisturbed values. This patch adds a SchedRead
that gets passed to MASK pseudos to represent this extra read that
must occur.

A future patch should do something similiar for when a pseudo is TU,
since those instructions must also read their destination to preserve
undisturbed-ness.

Differential Revision: https://reviews.llvm.org/D155654