[AArch64][DAGCombiner]: combine <2xi64> mul add/sub.

64-bit vector mul is not supported in NEON,
so we use the SVE's mul.
To improve the performance, we can go one step further,
and use SVE's add/sub, so that we can use SVE's mla/mls.
That works on these patterns:
add v1, (mul v2, v3)
sub v1, (mul v2, v3)

Reviewed By: david-arm, paulwalker-arm

Differential Revision: https://reviews.llvm.org/D148118