cmd/compile: add 32 bit (AddPtr (Const)) rule

This triggers about 50k times during 32 bit make.bash.

Change-Id: Ia0c2b1a8246b92173b4b0d94a4037626f76b6e73
Reviewed-on: https://go-review.googlesource.com/37998
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -98,6 +98,7 @@
         (Const32F [f2i(float64(i2f32(c) + i2f32(d)))]) // ensure we combine the operands with 32 bit precision
 (Add64F (Const64F [c]) (Const64F [d])) -> (Const64F [f2i(i2f(c) + i2f(d))])
 (AddPtr <t> x (Const64 [c])) -> (OffPtr <t> x [c])
+(AddPtr <t> x (Const32 [c])) -> (OffPtr <t> x [c])
 
 (Sub8   (Const8 [c]) (Const8 [d]))     -> (Const8 [int64(int8(c-d))])
 (Sub16  (Const16 [c]) (Const16 [d]))   -> (Const16 [int64(int16(c-d))])

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

@@ -1823,6 +1823,23 @@ func rewriteValuegeneric_OpAddPtr(v *Value, config *Config) bool {
 		v.AddArg(x)
 		return true
 	}
+	// match: (AddPtr <t> x (Const32 [c]))
+	// cond:
+	// result: (OffPtr <t> x [c])
+	for {
+		t := v.Type
+		x := v.Args[0]
+		v_1 := v.Args[1]
+		if v_1.Op != OpConst32 {
+			break
+		}
+		c := v_1.AuxInt
+		v.reset(OpOffPtr)
+		v.Type = t
+		v.AuxInt = c
+		v.AddArg(x)
+		return true
+	}
 	return false
 }
 func rewriteValuegeneric_OpAnd16(v *Value, config *Config) bool {