regalloc: optimize a little edge case better

With two-address instructions one needs to make sure the dst doesn't get
allocated to the same reg as the right-hand operand:

    %r = mul %x, %y  ; %y cannot be %r

Except, if the operands are the same

    %r = mul %x, %x  ; if %x is dead after this instr, it's fine to
                       allocate %r to the same reg

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