cmd/compile: constant-fold loads from constant dictionaries and types

Retrying the original CL with a small modification. The original CL
did not handle the case of reading an itab out of a dictionary
correctly.  When we read an itab out of a dictionary, we must treat
the type inside that itab as maybe being put in an interface.

Original CL: 486895
Revert CL: 490156

Change-Id: Id2dc1699d184cd8c63dac83986a70b60b4e6cbd7
Reviewed-on: https://go-review.googlesource.com/c/go/+/491495
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/internal/obj/link.go

@@ -465,7 +465,7 @@ type LSym struct {
 	P      []byte
 	R      []Reloc
 
-	Extra *interface{} // *FuncInfo, *VarInfo, or *FileInfo, if present
+	Extra *interface{} // *FuncInfo, *VarInfo, *FileInfo, or *TypeInfo, if present
 
 	Pkg    string
 	PkgIdx int32
@@ -588,6 +588,22 @@ func (s *LSym) File() *FileInfo {
 	return f
 }
 
+// A TypeInfo contains information for a symbol
+// that contains a runtime._type.
+type TypeInfo struct {
+	Type interface{} // a *cmd/compile/internal/types.Type
+}
+
+func (s *LSym) NewTypeInfo() *TypeInfo {
+	if s.Extra != nil {
+		panic(fmt.Sprintf("invalid use of LSym - NewTypeInfo with Extra of type %T", *s.Extra))
+	}
+	t := new(TypeInfo)
+	s.Extra = new(interface{})
+	*s.Extra = t
+	return t
+}
+
 // WasmImport represents a WebAssembly (WASM) imported function with
 // parameters and results translated into WASM types based on the Go function
 // declaration.