cmd/compile: fix compilation of math.Sqrt when used as a statement

Fixes #16804

Change-Id: I669c2c24d3135cd35e15a464894ac66945847d0c
Reviewed-on: https://go-review.googlesource.com/27437Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -966,6 +966,9 @@ func (s *state) stmt(n *Node) {
 		p := s.expr(n.Left)
 		s.nilCheck(p)
 
+	case OSQRT:
+		s.expr(n.Left)
+
 	default:
 		s.Unimplementedf("unhandled stmt %s", n.Op)
 	}

test/fixedbugs/issue16804.go

+// compile
+
+// Copyright 2016 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.
+
+// Issue 16804: internal error for math.Sqrt as statement
+//              rather than expression
+
+package main
+
+import "math"
+
+func sqrt() {
+	math.Sqrt(2.0)
+}