xtensa: Defer storing integer constants into litpool until reload

xtensa: Defer storing integer constants into litpool until reload

Storing integer constants into litpool in the early stage of compilation
hinders some integer optimizations.  In fact, such integer constants are
not subject to the constant folding process.

For example:

    extern unsigned short value;
    extern void foo(void);
    void test(void) {
      if (value == 30001)
        foo();
    }

	.literal_position
	.literal .LC0, value
	.literal .LC1, 30001
    test:
	l32r	a3, .LC0
	l32r	a2, .LC1
	l16ui	a3, a3, 0
	extui	a2, a2, 0, 16  // runtime zero-extension despite constant
	bne	a3, a2, .L1
	j.l	foo, a9
    .L1:
	ret.n

This patch defers the placement of integer constants into litpool until
the start of reload:

	.literal_position
	.literal .LC0, value
	.literal .LC1, 30001
    test:
	l32r	a3, .LC0
	l32r	a2, .LC1
	l16ui	a3, a3, 0
	bne	a3, a2, .L1
	j.l	foo, a9
    .L1:
	ret.n

gcc/ChangeLog:

	* config/xtensa/constraints.md (Y):
	Change to include integer constants until reload begins.
	* config/xtensa/predicates.md (move_operand): Ditto.
	* config/xtensa/xtensa.cc (xtensa_emit_move_sequence):
	Change to allow storing integer constants into litpool only after
	reload begins.