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, 10 insertions, 5 deletions

diff --git a/src/tools/wasm-reduce.cpp b/src/tools/wasm-reduce.cpp
index 029b9092d..e0491c0db 100644
--- a/src/tools/wasm-reduce.cpp
+++ b/src/tools/wasm-reduce.cpp
@@ -578,9 +578,11 @@ struct Reducer
           continue; // no conversion
         }
         Expression* fixed = nullptr;
-        switch (curr->type.getSingle()) {
+        TODO_SINGLE_COMPOUND(curr->type);
+        switch (curr->type.getBasic()) {
           case Type::i32: {
-            switch (child->type.getSingle()) {
+            TODO_SINGLE_COMPOUND(child->type);
+            switch (child->type.getBasic()) {
               case Type::i32:
                 WASM_UNREACHABLE("invalid type");
               case Type::i64:
@@ -605,7 +607,8 @@ struct Reducer
             break;
           }
           case Type::i64: {
-            switch (child->type.getSingle()) {
+            TODO_SINGLE_COMPOUND(child->type);
+            switch (child->type.getBasic()) {
               case Type::i32:
                 fixed = builder->makeUnary(ExtendSInt32, child);
                 break;
@@ -630,7 +633,8 @@ struct Reducer
             break;
           }
           case Type::f32: {
-            switch (child->type.getSingle()) {
+            TODO_SINGLE_COMPOUND(child->type);
+            switch (child->type.getBasic()) {
               case Type::i32:
                 fixed = builder->makeUnary(ConvertSInt32ToFloat32, child);
                 break;
@@ -655,7 +659,8 @@ struct Reducer
             break;
           }
           case Type::f64: {
-            switch (child->type.getSingle()) {
+            TODO_SINGLE_COMPOUND(child->type);
+            switch (child->type.getBasic()) {
               case Type::i32:
                 fixed = builder->makeUnary(ConvertSInt32ToFloat64, child);
                 break;