cmd/compile: add regress test for issue 19632

Updates #19632.

Change-Id: I1411dd997c8c6a789d17d0dcc0bfbd2281447b16
Reviewed-on: https://go-review.googlesource.com/38401
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue19632.go

+// compile
+
+// Copyright 2017 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.
+
+// Check that we don't crash due to "lost track of variable in
+// liveness" errors against unused variables.
+
+package p
+
+import "strings"
+
+// Minimized test case from github.com/mvdan/sh/syntax.
+func F() {
+	var _ = []string{
+		strings.Repeat("\n\n\t\t        \n", 10) +
+			"# " + strings.Repeat("foo bar ", 10) + "\n" +
+			strings.Repeat("longlit_", 10) + "\n",
+	}
+}