VOPD is a new encoding for dual-issue instructions for use in wave32.
This patch includes MC layer support only.
A VOPD instruction is constituted of an X component (for which there are
13 possible opcodes) and a Y component (for which there are the 13 X
opcodes plus 3 more). Most of the complexity in defining and parsing
a VOPD operation arises from the possible different total numbers of
operands and deferred parsing of certain operands depending on the
constituent X and Y opcodes.