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.txt

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