cmd/compile: use more specific error message for assignment mismatch

Show a more specifc error message in the form of "%d variables but %v
returns %d values" if an assignment mismatch occurs with a function
or method call on the right.

Fixes #27595

Change-Id: Ibc97d070662b08f150ac22d686059cf224e012ab
Reviewed-on: https://go-review.googlesource.com/135575
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

test/fixedbugs/issue27595.go

@@ -0,0 +1,19 @@
+// errorcheck
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+var a = twoResults()       // ERROR "assignment mismatch: 1 variable but twoResults returns 2 values"
+var b, c, d = twoResults() // ERROR "assignment mismatch: 3 variables but twoResults returns 2 values"
+var e, f = oneResult()     // ERROR "assignment mismatch: 2 variables but oneResult returns 1 values"
+
+func twoResults() (int, int) {
+	return 1, 2
+}
+
+func oneResult() int {
+	return 1
+}