arm: Add pattern for bswap + rotate -> rev16 [Bug 108933]

arm: Add pattern for bswap + rotate -> rev16 [Bug 108933]

The rev16 pattern was not recognised anymore as a change in the bswap
tree pass was introducing a new GIMPLE form, not recognized by the
assembly final transformation pass.

Also, fix the output patterns for arm_rev16si_alt[12] to correctly
handle the instructions being made conditional.

More details in the PR.

gcc/ChangeLog:

	PR target/108933
	* config/arm/arm.md (arm_rev16si2): Convert to define_insn.
	Correct generated RTL.
	(arm_rev16si2_alt1): Correctly handle conditional execution.
	(arm_rev16si2_alt2): Likewise.

gcc/testsuite/ChangeLog:

	PR target/108933
	* gcc.target/arm/rev16.c: Moved to...
	* gcc.target/arm/rev16_1.c: ...here.
	* gcc.target/arm/rev16_2.c: New test to check that rev16 is emitted.