fix read-decode information by latching not just the read reg number

fix read-decode information by latching not just the read reg number
but *whether it is valid or not*.
not only the register *number* from regspec_decode_read() needs to
be captured but its ok (valid) flag as well

this error only showed up when a mul (2-operand) was followed by addi
(1-operand).  the mul had its 2nd operand dropped on the floor because
the *addi* only has one operand.