Fix arc assumption that insns are not re-recognized

Fix arc assumption that insns are not re-recognized

Testing the V2 version of Manolis's fold-mem-offsets patch exposed a minor bug
in the arc backend.

The movsf_insn pattern has constraints which allow storing certain constants
to memory.  reload/lra will target those alternatives under the right
circumstances.  However the insn's condition requires that one of the two
operands must be a register.

Thus if a pass were to force re-recognition of the pattern we can get an
unrecognized insn failure.

This patch adjusts the conditions to more closely match movsi_insn.  More
specifically it allows storing a constant into a limited set of memory
operands (as defined by the Usc constraint).  movqi_insn has the same
core problem and gets the same solution.

Committed after the tester validated there are not regresisons

gcc/
	* config/arc/arc.md (movqi_insn): Allow certain constants to
	be stored into memory in the pattern's condition.
	(movsf_insn): Similarly.