test: extend noescape bytes.Buffer test suite

Added some more cases that should be guarded against regression.

Change-Id: I9f1dda2fd0be9b6e167ef1cc018fc8cce55c066c
Reviewed-on: https://go-review.googlesource.com/134017
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/fixedbugs/issue7921.go

@@ -37,3 +37,20 @@ func bufferNoEscape3(xs []string) string { // ERROR "xs does not escape"
 	}
 	return b.String() // ERROR "inlining call" "string\(bytes.b.buf\[bytes\.b\.off:\]\) escapes to heap"
 }
+
+func bufferNoEscape4() []byte {
+	var b bytes.Buffer
+	b.Grow(64)       // ERROR "b does not escape"
+	useBuffer(&b)    // ERROR "&b does not escape"
+	return b.Bytes() // ERROR "inlining call" "b does not escape"
+}
+
+func bufferNoEscape5() {
+	b := bytes.NewBuffer(make([]byte, 0, 128)) // ERROR "inlining call" "make\(\[\]byte, 0, 128\) does not escape" "&bytes.Buffer literal does not escape"
+	useBuffer(b)
+}
+
+//go:noinline
+func useBuffer(b *bytes.Buffer) { // ERROR "b does not escape"
+	b.WriteString("1234")
+}