(add/sub): Correct predicates and constraints for sNaddX and addX insns...

(add/sub): Correct predicates and constraints for sNaddX and addX insns...

(add/sub): Correct predicates and constraints for sNaddX and addX insns; we
can use sNsubX and subX to add small negative constants.
Don't split addition to SP since we want to update it just once.
(beq, bne): Use subtract, not compare for ints; it's more efficient.
(define_split for comparisons): Always prefer add/sub if it will work.
(probe_stack, allocate_stack): New patterns.

From-SVN: r4078