cmd/compile: avoid memmove -> SSA move rewrite when size is negative

We should panic in this situation. Rewriting to a SSA op just leads
to a compiler panic.

Fixes #36259

Change-Id: I6e0bccbed7dd0fdac7ebae76b98a211947947386
Reviewed-on: https://go-review.googlesource.com/c/go/+/212405
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

Author: randall77

src/cmd/compile/internal/ssa/gen/generic.rules

@@ -1938,6 +1938,7 @@
 
 // Inline small or disjoint runtime.memmove calls with constant length.
 (StaticCall {sym} s1:(Store _ (Const(64|32) [sz]) s2:(Store  _ src s3:(Store {t} _ dst mem))))
+	&& sz >= 0
 	&& isSameSym(sym,"runtime.memmove")
 	&& t.(*types.Type).IsPtr() // avoids TUINTPTR, see issue 30061
 	&& s1.Uses == 1 && s2.Uses == 1 && s3.Uses == 1

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

@@ -0,0 +1,28 @@
+// compile
+
+// Copyright 2019 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
+
+func rotate(s []int, m int) {
+    l := len(s)
+    m = m % l
+    buf := make([]int, m)
+
+    copy(buf, s)
+    copy(s, s[m:])
+    copy(s[l-m:], buf)
+}
+
+func main() {
+    a0 := [...]int{1,2,3,4,5}
+    println(a0[0])
+
+    rotate(a0[:], 1)
+    println(a0[0])
+
+    rotate(a0[:], -3)
+    println(a0[0])
+}