[llvm] [AArch64][GlobalISel] Add codegen for simd fpcvt intrinsics (PR #157680)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Thanks - high level I was wondering if we could remove aarch64_neon_fcvtzs entirely and just use fptosi_sat, but for strict-fp we likely still need it.

https://github.com/llvm/llvm-project/pull/157680