cmd/8g: roll back the small integer constant optimizations introduced

in 13416:67c0b8c8fb29 "faster code, mainly for rotate" [1]. The codegen
can run out of registers if there are too many small-int arithmetic ops.

An alternative approach is to copy 6g's sbop/abop codegen to 8g, but
this change is less risky.

Fixes #3835.

[1] http://code.google.com/p/go/source/diff?spec=svn67c0b8c8fb29b1b7b6221977af6b89cae787b941&name=67c0b8c8fb29&r=67c0b8c8fb29b1b7b6221977af6b89cae787b941&format=side&path=/src/cmd/8g/cgen.c

R=rsc, remyoudompheng, r
CC=golang-dev
https://golang.org/cl/6450163

src/cmd/8g/cgen.c

@@ -392,13 +392,7 @@ sbop:	// symmetric binary
 	}
 
 abop:	// asymmetric binary
-	if(smallintconst(nr)) {
-		regalloc(&n1, nr->type, res);
-		cgen(nl, &n1);
-		gins(a, nr, &n1);
-		gmove(&n1, res);
-		regfree(&n1);
-	} else if(nl->ullman >= nr->ullman) {
+	if(nl->ullman >= nr->ullman) {
 		tempname(&nt, nl->type);
 		cgen(nl, &nt);
 		mgen(nr, &n2, N);

test/fixedbugs/bug451.go

+// run
+
+// Copyright 2012 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 3835: 8g tries to optimize arithmetic involving integer
+// constants, but can run out of registers in the process.
+
+package main
+
+var a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z, A, B, C, D, E, F, G int
+
+func foo() int {
+	return a + 1 + b + 2 + c + 3 + d + 4 + e + 5 + f + 6 + g + 7 + h + 8 + i + 9 + j + 10 +
+		k + 1 + l + 2 + m + 3 + n + 4 + o + 5 + p + 6 + q + 7 + r + 8 + s + 9 + t + 10 +
+		u + 1 + v + 2 + w + 3 + x + 4 + y + 5 + z + 6 + A + 7 + B + 8 + C + 9 + D + 10 +
+		E + 1 + F + 2 + G + 3
+}
+
+func bar() int8 {
+	var (
+		W int16
+		X int32
+		Y int32
+		Z int32
+	)
+	return int8(W+int16(X+3)+3) * int8(Y+3+Z*3)
+}
+
+func main() {
+	if foo() == 0 {
+		panic("foo")
+	}
+	if bar() == 0 {
+		panic("bar")
+	}
+}