arm: Improve immediate generation for thumb-1 with -mpurecode [PR96769]

arm: Improve immediate generation for thumb-1 with -mpurecode [PR96769]

This patch moves the move-immediate splitter after the regular ones so
that it has lower precedence, and updates its constraints.

For
int f3 (void) { return 0x11000000; }
int f3_2 (void) { return 0x12345678; }

we now generate:
* with -O2 -mcpu=cortex-m0 -mpure-code:
f3:
	movs    r0, #136
	lsls    r0, r0, #21
	bx      lr
f3_2:
	movs    r0, #18
	lsls    r0, r0, #8
	adds    r0, r0, #52
	lsls    r0, r0, #8
	adds    r0, r0, #86
	lsls    r0, r0, #8
	adds    r0, r0, #121
	bx      lr

* with -O2 -mcpu=cortex-m23 -mpure-code:
f3:
	movs    r0, #136
	lsls    r0, r0, #21
	bx      lr
f3_2:
	movw    r0, #22136
	movt    r0, 4660
	bx      lr

2020-09-04  Christophe Lyon  <christophe.lyon@linaro.org>

	PR target/96769
	gcc/
	* config/arm/thumb1.md: Move movsi splitter for
	arm_disable_literal_pool after the other movsi splitters.

	gcc/testsuite/
	* gcc.target/arm/pure-code/pr96769.c: New test.