[clang codegen][AArch64] Use llvm.aarch64.neon.fcvtzs/u where it's necessary

fptosi/fptoui have similar, but not identical, semantics.  In
particular, the behavior on overflow is different.

Fixes https://bugs.llvm.org/show_bug.cgi?id=46844 for 64-bit.  (The
corresponding patch for 32-bit is more involved because the equivalent
intrinsics don't exist, as far as I can tell.)

Differential Revision: https://reviews.llvm.org/D84703