Reintroduce non-pow(2, N) SIMD vectors

Filing as requested in #1703.
#[repr(simd)] types with odd lane counts were supported by LLVM, which just used a vector of an appropriate size to include the lane count.

However, cg_clif does not currently support these, and as such they have been removed until https://github.com/bjorn3/rustc_codegen_cranelift/issues/1136 is addressed and this limitation has been removed from rustc.