Allow for instructions with operands in encodings.
Allow for instructions with operands in encodings.

When defining an instruction encoding, allow part of the instruction
predicate to be provided as operands on the instruction opcode:

    icmp.i32(intcc.ult, x, y)

This generates an instruction predicate that checks

    IntCompare.cond == IntCC::UnsignedLessThan

stoklund/cretonne,stoklund/cretonne,sunfishcode/cretonne,stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne