Hi all,

In the M-Class Arm-ARM:

https://developer.arm.com/documentation/ddi0553/bu/?lang=en

these MVE instructions only have '!' writeback variant and at:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107714

we found that the Um constraint would also allow through a
register offset writeback, resulting in an assembler error.

Here I have added a new constraint and predicate for these
instructions, which (uniquely, AFAICT), only support a `!` writeback
increment by the data size (inside the compiler this is a POST_INC).

No regressions in arm-none-eabi with MVE and MVE.FP.

Ok for trunk, and backport to GCC11 and GCC12 (testing pending)?

Thanks,
Stam

gcc/ChangeLog:
         PR target/107714
         * config/arm/arm-protos.h (mve_struct_mem_operand): New protoype.
         * config/arm/arm.cc (mve_struct_mem_operand): New function.
         * config/arm/constraints.md (Ug): New constraint.
         * config/arm/mve.md (mve_vst4q<mode>): Change constraint.
         (mve_vst2q<mode>): Likewise.
         (mve_vld4q<mode>): Likewise.
         (mve_vld2q<mode>): Likewise.
         * config/arm/predicates.md (mve_struct_operand): New predicate.

gcc/testsuite/ChangeLog:
         PR target/107714
         * gcc.target/arm/mve/intrinsics/vldst24q_reg_offset.c: New test.