[AArch64] Use custom lowering for {U,S}INT_TO_FP with i8.

With fullfp16, it is cheaper to cast the {U,S}INT_TO_FP operand to i16
first, rather than promoting it to i32. The custom lowering for
{U,S}INT_TO_FP  already supports that, it just needs to be used.

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D113601