cmd/compile: accept literals in samesafeexpr

This only triggers a few places in the stdlib,
but it helps a lot when it does.

Before:

runtime.mapassign1 t=1 size=2400 args=0x20 locals=0xe0

After:

runtime.mapassign1 t=1 size=2352 args=0x20 locals=0xd8

name           old time/op  new time/op  delta
MapPop100-8    19.8µs ±11%  18.4µs ± 9%  -7.16%  (p=0.000 n=20+19)
MapPop1000-8    367µs ±17%   335µs ±11%  -8.63%  (p=0.000 n=19+19)
MapPop10000-8  7.29ms ±15%  6.86ms ±12%  -5.84%  (p=0.020 n=20+20)

Change-Id: I9faf32f95a6ba6a6d5d0818eab32cc271e01d10a
Reviewed-on: https://go-review.googlesource.com/26666
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -61,6 +61,34 @@ func (v Val) Ctype() Ctype {
 	}
 }
 
+func eqval(a, b Val) bool {
+	if a.Ctype() != b.Ctype() {
+		return false
+	}
+	switch x := a.U.(type) {
+	default:
+		Fatalf("unexpected Ctype for %T", a.U)
+		panic("not reached")
+	case *NilVal:
+		return true
+	case bool:
+		y := b.U.(bool)
+		return x == y
+	case *Mpint:
+		y := b.U.(*Mpint)
+		return x.Cmp(y) == 0
+	case *Mpflt:
+		y := b.U.(*Mpflt)
+		return x.Cmp(y) == 0
+	case *Mpcplx:
+		y := b.U.(*Mpcplx)
+		return x.Real.Cmp(&y.Real) == 0 && x.Imag.Cmp(&y.Imag) == 0
+	case string:
+		y := b.U.(string)
+		return x == y
+	}
+}
+
 type NilVal struct{}
 
 // IntLiteral returns the Node's literal value as an integer.

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

@@ -3185,6 +3185,9 @@ func samesafeexpr(l *Node, r *Node) bool {
 
 	case OINDEX:
 		return samesafeexpr(l.Left, r.Left) && samesafeexpr(l.Right, r.Right)
+
+	case OLITERAL:
+		return eqval(l.Val(), r.Val())
 	}
 
 	return false