ir: fix interface call edge case

A function on a type that is put in an interface must not be assumed to
not need a context pointer when it isn't directly put in a function
pointer, because the interface call side won't know this and pass an
extra parameter with that extra function pointer.

This is usually not a problem but WebAssembly has strict checks on
function signatures so this resulted in an error when running
src/examples/test/test.go.

ir/passes.go

@@ -416,6 +416,12 @@ func (p *Program) AllInterfaces() []*Interface {
 
 func (p *Program) FunctionNeedsContext(f *Function) bool {
 	if !f.addressTaken {
+		if f.Signature.Recv() != nil {
+			_, hasInterfaceConversion := p.TypeNum(f.Signature.Recv().Type())
+			if hasInterfaceConversion && p.SignatureNeedsContext(f.Signature) {
+				return true
+			}
+		}
 		return false
 	}
 	return p.SignatureNeedsContext(f.Signature)