This follows the pattern of lowering VP nodes to equivalent
RISCVISD::*_VL nodes. The nodes are modelled after the VP ISD nodes rather
than the actual zvbb instructions, and I've included a merge operand to be
consistent with the underlying pseudos that were recently refactored.
I've defined the nodes in RISCVInstrInfoVVLpatterns.td as the nodes aren't Zvk
specific, but the patterns are in RISCVInstrInfoZvk.td.
This has the same predicate problem?