Implement vector element extensions for AArch64

This commit also includes load and extend operations. Both are
prerequisites for enabling further SIMD spec tests.

Copyright (c) 2020, Arm Limited.

cranelift/codegen/src/isa/aarch64/abi.rs

@@ -406,7 +406,7 @@ fn in_int_reg(ty: ir::Type) -> bool {
 
 fn in_vec_reg(ty: ir::Type) -> bool {
     match ty {
-        types::F32 | types::F64 | types::I8X16 => true,
+        types::F32 | types::F64 | types::I8X16 | types::I16X8 | types::I32X4 | types::I64X2 => true,
         _ => false,
     }
 }