[CostModel][AArch64] Remove amortization factor for some of the vector select instructions

Summary:
We are not scalarizing the wide selects in codegen for i16 and i32 and
therefore we can remove the amortization factor. We still have issues
with i64 vectors in codegen though.

Reviewers: mcrosier

Subscribers: mcrosier, aemerson, llvm-commits, rengolin

Differential Revision: http://reviews.llvm.org/D12724

llvm-svn: 247156