gc: better error message for bad type in channel send

# test program
1 package main
2
3 type C chan int
4
5 func F(c C) {
6 	c <- true
7 }

# old error
test.go:6: cannot use true (type bool) as type int in function argument

# new error
test.go:6: cannot use true (type bool) as type int in channel send

R=rsc, ejsherry
CC=golang-dev
https://golang.org/cl/3231042