compiler: avoid bitcast when replacing a method call with a direct call

A bitcast was inserted when the receiver of the call wasn't a *i8. This
is a pretty common case, and did not play well with goroutines.
Avoid this bitcast by changing each call to a direct call, after
unpacking the receiver type from the *i8 parameter. This might also fix
some undefined behavior in the resulting program, as it is technically
not allowed to call a function with a different signature (even if the
signature is compatible).

testdata/coroutines.go

@@ -21,6 +21,10 @@ func main() {
 	go nowait()
 	time.Sleep(time.Millisecond)
 	println("done with non-blocking goroutine")
+
+	var printer Printer
+	printer = &myPrinter{}
+	printer.Print()
 }
 
 func sub() {
@@ -38,3 +42,15 @@ func wait() {
 func nowait() {
 	println("non-blocking goroutine")
 }
+
+type Printer interface {
+	Print()
+}
+
+type myPrinter struct{
+}
+
+func (i *myPrinter) Print() {
+	time.Sleep(time.Millisecond)
+	println("async interface method call")
+}

testdata/coroutines.txt

@@ -9,3 +9,4 @@ wait:
 end waiting
 non-blocking goroutine
 done with non-blocking goroutine
+async interface method call