Prepare for compound types that are single but not basic (#3046)

As a follow-up to https://github.com/WebAssembly/binaryen/pull/3012#pullrequestreview-459686171 this PR prepares for the new compound Signature, Struct and Array types that are single but not basic.

This includes:

* Renames `Type::getSingle` to `Type::getBasic` (NFC). Previously, its name was not representing its implementation (`isSingle` excluded `none` and `unreachable` while `getSingle` didn't, i.e. `getSingle` really was `getBasic`). Note that a hypothetical `Type::getSingle` cannot return `ValueType` anyway (new compound types are single but don't map to `ValueType`), so I figured it's best to skip implementing it until we actually need it.
* Marks locations where we are (still) assuming that all single types are basic types, as suggested in https://github.com/WebAssembly/binaryen/pull/3012#discussion_r465356708, but using a macro, so we get useful errors once we start implementing the new types and can quickly traverse the affected locations.

The macro is added where

* there used to be a `switch (type.getSingle())` or similar that handled any basic type (NFC), but in the future will also have to handle single types that are not basic types.
* we are not dealing with `Unary`, `Binary`, `Load`, `Store` or `AtomicXY` instructions, since these don't deal with compound types anyway.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/asmjs/asm_v_wasm.cpp b/src/asmjs/asm_v_wasm.cpp
index 98a6be046..75fc0506b 100644
--- a/src/asmjs/asm_v_wasm.cpp
+++ b/src/asmjs/asm_v_wasm.cpp
@@ -42,7 +42,8 @@ Type asmToWasmType(AsmType asmType) {
 }
 
 AsmType wasmToAsmType(Type type) {
-  switch (type.getSingle()) {
+  TODO_SINGLE_COMPOUND(type);
+  switch (type.getBasic()) {
     case Type::i32:
       return ASM_INT;
     case Type::f32:
@@ -67,7 +68,8 @@ AsmType wasmToAsmType(Type type) {
 }
 
 char getSig(Type type) {
-  switch (type.getSingle()) {
+  TODO_SINGLE_COMPOUND(type);
+  switch (type.getBasic()) {
     case Type::i32:
       return 'i';
     case Type::i64: