arm: MVE: Relax addressing modes for full loads and stores

arm: MVE: Relax addressing modes for full loads and stores

This patch relaxes the addressing modes for the mve full load and stores (by
full loads and stores I mean non-widening or narrowing loads and stores resp).
The code before was requiring a LO_REGNUM for these, where this is only a
requirement if the load is widening or the store narrowing.

gcc/ChangeLog:

	PR target/104790
	* config/arm/arm.h (MVE_STN_LDW_MODE): New MACRO.
	* config/arm/arm.cc (mve_vector_mem_operand): Relax constraint on base
	register for non widening loads or narrowing stores.