[PATCH] rx: avoid adding setpsw for rx_cmpstrn when len is const

[PATCH] rx: avoid adding setpsw for rx_cmpstrn when len is const

pattern using rx_cmpstrn is cmpstrsi for which len is a constant -1,
so we'll be moving the setpsw instructions from rx_cmpstrn to
cmpstrnsi as follows:

 1. Adjust the predicate on the length operand from "register_operand"
    to "nonmemory_operand". This will allow constants to appear here,
    instead of having them already transferred into a register.

 2. Check to see if the len value is constant, and then check if it is
    actually zero. In that case, short-circuit the rest of the pattern
    and set the result register to 0.

 3. Emit 'setpsw c' and 'setpsw z' instructions when the len is not a
    constant, in case it turns out to be zero at runtime.

 4. Remove the two 'setpsw' instructions from rx_cmpstrn.

gcc/
	* config/rx/rx.md (cmpstrnsi): Allow constant length.  For
	static length 0, just store 0 into the output register.
	For dynamic zero, set C/Z appropriately.
	(rxcmpstrn): No longer set C/Z.