arm/98435: Missed optimization in expanding vector constructor.

arm/98435: Missed optimization in expanding vector constructor.

The patch moves vec_init pattern from neon.md to vec-common.md,
and adjusts the mode to VDQX to accomodate binary floats. Also,
the pattern is additionally gated on VALID_MVE_MODE.

gcc/ChangeLog:
	PR target/98435
	* config/arm/neon.md (vec_init): Move to ...
	* config/arm/vec-common.md (vec_init): ... here.
	Change the pattern's mode to VDQX and gate it on VALID_MVE_MODE.

gcc/testsuite/ChangeLog:
	PR target/98435
	* gcc.target/arm/simd/pr98435.c: New test.