[AArch64] Lower fpto*i.sat intrinsics.

AArch64's fctv* instructions implement the saturating behaviour that the
fpto*i.sat intrinsics require, in cases where the destination width
matches the saturation width. Lowering them removes a lot of unnecessary
generated code.

Only scalar lowerings are supported for now.

Differential Revision: https://reviews.llvm.org/D102353