xtensa: constantsynth: Make try to find shorter instruction

xtensa: constantsynth: Make try to find shorter instruction

This patch allows the constant synthesis to choose shorter instruction
if possible.

    /* example */
    int test(void) {
      return 128 << 8;
    }

    ;; before
    test:
	movi	a2, 0x100
	addmi	a2, a2, 0x7f00
	ret.n

    ;; after
    test:
	movi.n	a2, 1
	slli	a2, a2, 15
	ret.n

When the Code Density Option is configured, the latter is one byte smaller
than the former.

gcc/ChangeLog:

	* config/xtensa/xtensa.cc (xtensa_emit_constantsynth): Remove.
	(xtensa_constantsynth_2insn): Change to try all three synthetic
	methods and to use the one that fits the immediate value of
	the seed into a Narrow Move Immediate instruction "MOVI.N"
	when the Code Density Option is configured.