[RISCV] Common post-mask operand construction 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.

This particular change involves a slightly ugly bit of code to match the glue to the mask.  I'm staging it this way as I ran into a bit of weirdness when commoning mask operands, and wanted to isolate the complexity.