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.

More details in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108933

gcc/ChangeLog:

         PR target/108933
         * config/arm/arm.md (*arm_rev16si2_alt3): new pattern to convert
           a bswap + rotate by 16 bits into rev16

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.