arm64: Refactor FMA simplification.

Move it from the generic instruction simplifier to the arm64
simplifier. Do the simplification while visiting `VecAdd` or
`VecSub`, not while visiting `VecMul`.

Test: testrunner.py --target --64 --optimizing
Bug: 181943478
Flag: EXEMPT refactor
Change-Id: Ib63ae4c4fd6df55e515436ca51ab43eb0730ebd1
3 files changed