cmd/compile: remove unnecessary convert ops on 32-bit archs

Missing rule to fold out Convert ops on 32-bit architectures.

This comes up with expressions like
unsafe.Pointer(uintptr(p) + x)

Change-Id: I429e968e5d1a3e13a386ddd29a08ebb6d7dd938a
Reviewed-on: https://go-review.googlesource.com/112158
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -859,7 +859,7 @@
 (If (ConstBool [c]) yes no) && c == 0 -> (First nil no yes)
 
 // Get rid of Convert ops for pointer arithmetic on unsafe.Pointer.
-(Convert (Add64 (Convert ptr mem) off) mem) -> (Add64 ptr off)
+(Convert (Add(64|32) (Convert ptr mem) off) mem) -> (Add(64|32) ptr off)
 (Convert (Convert ptr mem) mem) -> ptr
 
 // Decompose compound argument values

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

@@ -7489,6 +7489,58 @@ func rewriteValuegeneric_OpConvert_0(v *Value) bool {
 		v.AddArg(off)
 		return true
 	}
+	// match: (Convert (Add32 (Convert ptr mem) off) mem)
+	// cond:
+	// result: (Add32 ptr off)
+	for {
+		_ = v.Args[1]
+		v_0 := v.Args[0]
+		if v_0.Op != OpAdd32 {
+			break
+		}
+		_ = v_0.Args[1]
+		v_0_0 := v_0.Args[0]
+		if v_0_0.Op != OpConvert {
+			break
+		}
+		_ = v_0_0.Args[1]
+		ptr := v_0_0.Args[0]
+		mem := v_0_0.Args[1]
+		off := v_0.Args[1]
+		if mem != v.Args[1] {
+			break
+		}
+		v.reset(OpAdd32)
+		v.AddArg(ptr)
+		v.AddArg(off)
+		return true
+	}
+	// match: (Convert (Add32 off (Convert ptr mem)) mem)
+	// cond:
+	// result: (Add32 ptr off)
+	for {
+		_ = v.Args[1]
+		v_0 := v.Args[0]
+		if v_0.Op != OpAdd32 {
+			break
+		}
+		_ = v_0.Args[1]
+		off := v_0.Args[0]
+		v_0_1 := v_0.Args[1]
+		if v_0_1.Op != OpConvert {
+			break
+		}
+		_ = v_0_1.Args[1]
+		ptr := v_0_1.Args[0]
+		mem := v_0_1.Args[1]
+		if mem != v.Args[1] {
+			break
+		}
+		v.reset(OpAdd32)
+		v.AddArg(ptr)
+		v.AddArg(off)
+		return true
+	}
 	// match: (Convert (Convert ptr mem) mem)
 	// cond:
 	// result: ptr