[AArch64] Add instruction costs for FP_TO_UINT and FP_TO_SINT with half types

We were missing some instruction costs when converting vectors of
floating point half types into integers, so I've added those here.
I also manually generated assembly code for each FP->int case and
looked at the number of instructions generated, which meant
adjusting some of the existing costs too.

I've updated an existing test to reflect the new costs:

  Analysis/CostModel/AArch64/sve-fptoi.ll

Differential Revision: https://reviews.llvm.org/D99935