AArch64: Remove redundant check in aarch64_simd_mov

The split condition in aarch64_simd_mov uses aarch64_simd_special_constant_p.
While doing the split, it checks the mode before calling
aarch64_maybe_generate_simd_constant.  This risky since it may result in
unexpectedly calling aarch64_split_simd_move instead of
aarch64_maybe_generate_simd_constant.  Since the mode is already checked,
remove the spurious explicit mode check.

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md (aarch64_simd_mov<VQMOV:mode>):
	Remove redundant mode check.