Instruction selection: multiple patterns for single instruction?

Hi everyone,

I'm trying to add intrinsics for some already-existing instructions in
the MIPS target. How can I make so that a single instruction is selected
for multiple possible patterns?

For example,

    class shift_rotate_imm<string opstr, Operand ImmOpnd,
                          RegisterOperand RO, InstrItinClass itin,
                          SDPatternOperator OpNode = null_frag,
                          SDPatternOperator PF = null_frag> :
      InstSE<(outs RO:$rd), (ins RO:$rt, ImmOpnd:$shamt),
            !strconcat(opstr, "\t$rd, $rt, $shamt"),
            [(set RO:$rd, (OpNode RO:$rt, PF:$shamt))],
            itin, FrmR, opstr> {
      let TwoOperandAliasConstraint = "$rt = $rd";

Is selected for the pattern

   (set RO:$rd, (OpNode RO:$rt, PF:$shamt))

How can I make so that the instructions instantiated from this class are
also selected for

  (set RO:$rd, (int_mips_my_builtin RO:$rt, PF:$shamt))

? Appending the new pattern to the existing list obviously won't work
since that is used for (AFAIU) a sequence of operations.