[RISCV] Reason explicitly about mask and rounding mode in performCombineVMergeAndVOps...

[RISCV] Reason explicitly about mask and rounding mode 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.

The code structure here is overly verbose.  I'm landing this staging change with the code structure exactly matching the non-masked case to make the following cleanup that commons this all obviously correct.