[AArch64][SVE] Lower MULHU/MULHS nodes to umulh/smulh instructions

Mark MULHS/MULHU nodes as legal for both scalable and fixed SVE types,
and lower them to the appropriate SVE instructions.

Additionally now that the MULH nodes are legal, integer divides can be
expanded into a more performant code sequence.

Differential Revision: https://reviews.llvm.org/D100487