[arm] Fix MVE addressing modes for VLDR[BHW] and VSTR[BHW]

[arm] Fix MVE addressing modes for VLDR[BHW] and VSTR[BHW]

The way we were previously dealing with addressing modes for MVE was preventing
the use of pre, post and offset addressing modes for the normal loads and
stores, including widening and narrowing.  This patch fixes that and
adds tests to ensure we are capable of using all the available addressing
modes.

gcc/ChangeLog:
2021-10-12  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* config/arm/arm.c (thumb2_legitimate_address_p): Use VALID_MVE_MODE
	when checking mve addressing modes.
	(mve_vector_mem_operand): Fix the way we handle pre, post and offset
	addressing modes.
	(arm_print_operand): Fix printing of POST_ and PRE_MODIFY.
	* config/arm/mve.md: Use mve_memory_operand predicate everywhere where
	there is a single Ux constraint.

gcc/testsuite/ChangeLog:
2021-10-12  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* gcc.target/arm/mve/mve.exp: Make it test main directory.
	* gcc.target/arm/mve/mve_load_memory_modes.c: New test.
	* gcc.target/arm/mve/mve_store_memory_modes.c: New test.