arm: mve: fix auto-inc generation [PR107674]

My change r13-416-g485a0ae0982abe caused the compiler to stop
generating auto-inc operations on mve loads and stores.  The fix
is to check whether there is a replacement register available
when in strict mode and the register is still a pseudo.

gcc:

	PR target/107674
	* config/arm/arm.cc (arm_effective_regno): New function.
	(mve_vector_mem_operand): Use it.