[AArch64][CostModel] Fix cost for mul <2 x i64>
This was modeled to have a cost of 1, but since we do not have a MUL.2d this is
scalarized into vector inserts/extracts and scalar muls.
Motivating precommitted test is test/Transforms/SLPVectorizer/AArch64/mul.ll,
which we don't want to SLP vectorize.
unfortunately needed changing, but the reason is documented in
// The cost of executing VF copies of the scalar instruction. This opcode // is unknown. Assume that it is the same as 'mul'.
which I will address next as a follow up of this.
Differential Revision: https://reviews.llvm.org/D92208