xtensa: Add 'subtraction from constant' insn pattern

This patch makes try to eliminate using temporary pseudo for
'(minus:SI (const_int) (reg:SI))' if the addition of negative constant
value can be emitted in a single machine instruction.

    /* example */
    int test0(int x) {
      return 1 - x;
    }
    int test1(int x) {
      return 100 - x;
    }
    int test2(int x) {
      return 25600 - x;
    }

    ;; before
    test0:
	movi.n	a9, 1
	sub	a2, a9, a2
	ret.n
    test1:
	movi	a9, 0x64
	sub	a2, a9, a2
	ret.n
    test2:
	movi.n	a9, 0x19
	slli	a9, a9, 10
	sub	a2, a9, a2
	ret.n

    ;; after
    test0:
	addi.n	a2, a2, -1
	neg	a2, a2
	ret.n
    test1:
	addi	a2, a2, -100
	neg	a2, a2
	ret.n
    test2:
	addmi	a2, a2, -0x6400
	neg	a2, a2
	ret.n

    gcc/ChangeLog:

	* config/xtensa/xtensa-protos.h (xtensa_m1_or_1_thru_15):
	New prototype.
	* config/xtensa/xtensa.cc (xtensa_m1_or_1_thru_15):
	New function.
	* config/xtensa/constraints.md (O):
	Change to use the above function.
	* config/xtensa/xtensa.md (*subsi3_from_const):
	New insn_and_split pattern.

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