xtensa: Make use of CLAMPS instruction if configured

xtensa: Make use of CLAMPS instruction if configured

This patch introduces the use of CLAMPS instruction when the instruction
is configured.

    /* example */
    int test(int a) {
      if (a < -512)
        return -512;
      if (a > 511)
        return 511;
      return a;
    }

    ;; prereq: TARGET_CLAMPS
    test:
	clamps	a2, a2, 9
	ret.n

gcc/ChangeLog:

	* config/xtensa/xtensa-protos.h (xtensa_match_CLAMPS_imms_p):
	New prototype.
	* config/xtensa/xtensa.cc (xtensa_match_CLAMPS_imms_p):
	New function.
	* config/xtensa/xtensa.h (TARGET_CLAMPS): New macro definition.
	* config/xtensa/xtensa.md (*xtensa_clamps): New insn pattern.