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 no regressions with this 
patch installed for arc-elf.

Jeff