[RISCV] Simplify glue handling logic in performCombineVMergeAndVOps [nfc]

This is a subset of Luke's D155063.  I'm splitting pieces and landing them in the process of convincing myself all the individual transforms are in fact correct.

In this case, we're simplifying based on the assumption that all of our vmerge operands have mask operands.  This is a fundemental property of a vmerge.