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

[AArch64][DAGCombiner]: combine <2xi64> 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:
// This works on the patterns of:
//   add v1, (mul v2, v3)
//   sub v1, (mul v2, v3)

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D147236