cg_llvm: scalable vectors with `simd_cast` and `simd_select` Previously `sve_cast`'s implementation was abstracted to power both `sve_cast` and `simd_cast` which supported scalable and non-scalable vectors respectively. In anticipation of having to do this for another `simd_*` intrinsic, `sve_cast` is removed and `simd_cast` is changed to accept both scalable and non-scalable intrinsics, an approach that will scale better to the other intrinsics. Building on the previous change, support scalable vectors with `simd_select`. Previous patches already landed the necessary changes in the implementation of this intrinsic, but didn't allow scalable vector arguments to be passed in.