RISC-V: match_*() improvements

Match functions referenced by macro insns should not
unconditionally invoke match_opcode(): The macro enumeration can change
(grow), making the function potentially yield false negatives.

In match_rs1_nonzero(), used solely by macro insns, add an assertion to
that effect.

While there also drop the pointless attribute from
match_rs1_nonzero_rs2_even()'s first parameter.