[AArch64 obvious] Fix register constraints for aarch64_ml[as]_elt_merge<mode>

[AArch64 obvious] Fix register constraints for aarch64_ml[as]_elt_merge<mode>

The MLA by-element instructions have the same restriction as other by-element
instructions whereby the forms operating on vectors of 16-bit integer data
may only use registers v0-v15. We have an iterator for that, applied to the
other patterns generating this instruction, so use that.

gcc/

	* config/aarch64/aarch64-simd.md (aarch64_mla_elt_merge<mode>): Fix
	register constraint for by-element operand.
	(aarch64_mls_elt_merge<mode>): Likewise.

From-SVN: r251568