compiler: fix equally named structs in different scopes

For example, in this code:

    type kv struct {
           v float32
    }

    func foo(a *kv) {
           type kv struct {
                   v byte
           }
    }

Both 'kv' types would be given the same LLVM type, even though they are
different types! This is fixed by only creating a LLVM type once per Go
type (types.Type).

As an added bonus, this change gives a performance improvement of about
0.4%. Not that much, but certainly not nothing for such a small change.

compiler/testdata/basic.go

@@ -55,3 +55,18 @@ func complexMul(x, y complex64) complex64 {
 }
 
 // TODO: complexDiv (requires runtime call)
+
+// A type 'kv' also exists in function foo. Test that these two types don't
+// conflict with each other.
+type kv struct {
+	v float32
+}
+
+func foo(a *kv) {
+	// Define a new 'kv' type.
+	type kv struct {
+		v byte
+	}
+	// Use this type.
+	func(b *kv) {}(nil)
+}