[AArch64][GlobalISel] Add G_SQDMULL node
Previously, GISel was failing to lower the sqdmulls.scalar intrinsic. This is just a variation of sqdmull, but on two 32-bit S registers.
To fix this, create a G_SQDMULL node, and lower sqdmulls.scalar to that. This node is linked to the SD patterns for sqdmull, which allow this version of the intrinsic to lower.
