cmd/compile: avoid an assignment of n.Type in walk

In the future, walk will probably run concurrently
with SSA construction. It is possible for walk
to be walking a function node that is referred
to by another function undergoing SSA construction.
In that case, this particular assignment to n.Type
is race-y.

This assignment is also not necessary;
evconst does not change the type of n.
Both arguments to evconst must have the same type,
and at the end of evconst, n is replaced with n.Left.

Remove the assignment, and add a check to ensure
that its removal remains correct.

Updates #15756

Change-Id: Id95faaff42d5abd76be56445d1d3e285775de8bf
Reviewed-on: https://go-review.googlesource.com/38609
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -1614,9 +1614,10 @@ opswitch:
 	// walk of y%1 may have replaced it by 0.
 	// Check whether n with its updated args is itself now a constant.
 	t := n.Type
-
 	evconst(n)
-	n.Type = t
+	if n.Type != t {
+		Fatalf("evconst changed Type: %v had type %v, now %v", n, t, n.Type)
+	}
 	if n.Op == OLITERAL {
 		n = typecheck(n, Erv)
 	}