[dev.regabi] cmd/compile: simplify ir.Find, replace ir.Inspect with ir.Visit

It seems clear after using these for a week that Find need not return
anything other than a bool saying whether the target was found.
The main reason for not using the boolean earlier was to avoid confusion
with Inspect: for Find, returning true means "it was found! stop walking"
while for Inspect, returning true means "keep walking the children".

But it turns out that none of the uses of Inspect need the boolean.
This makes sense because types can contain expressions, expressions
can contain statements (inside function literals), and so on, so there
are essentially no times when you can say based on the current AST node
that the children are irrelevant to a particular operation.

So this CL makes two changes:

1) Change Find to return a boolean and to take a callback function
returning a boolean. This simplifies all existing calls to Find.

2) Rename Inspect to Visit and change it to take a callback with no
result at all. This simplifies all existing calls to Inspect.

Removing the boolean result from Inspect's callback avoids having
two callbacks with contradictory boolean results in different APIs.
Renaming Inspect to Visit avoids confusion with ast.Inspect.

Passes buildall w/ toolstash -cmp.

Change-Id: I344ebb5e00b6842012be33e779db483c28e5f350
Reviewed-on: https://go-review.googlesource.com/c/go/+/277919
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -3764,11 +3764,11 @@ func usefield(n ir.Node) {
 
 // hasSideEffects reports whether n contains any operations that could have observable side effects.
 func hasSideEffects(n ir.Node) bool {
-	found := ir.Find(n, func(n ir.Node) interface{} {
+	return ir.Find(n, func(n ir.Node) bool {
 		switch n.Op() {
 		// Assume side effects unless we know otherwise.
 		default:
-			return n
+			return true
 
 		// No side effects here (arguments are checked separately).
 		case ir.ONAME,
@@ -3824,29 +3824,28 @@ func hasSideEffects(n ir.Node) bool {
 			ir.OREAL,
 			ir.OIMAG,
 			ir.OCOMPLEX:
-			return nil
+			return false
 
 		// Only possible side effect is division by zero.
 		case ir.ODIV, ir.OMOD:
 			if n.Right().Op() != ir.OLITERAL || constant.Sign(n.Right().Val()) == 0 {
-				return n
+				return true
 			}
 
 		// Only possible side effect is panic on invalid size,
 		// but many makechan and makemap use size zero, which is definitely OK.
 		case ir.OMAKECHAN, ir.OMAKEMAP:
 			if !ir.IsConst(n.Left(), constant.Int) || constant.Sign(n.Left().Val()) != 0 {
-				return n
+				return true
 			}
 
 		// Only possible side effect is panic on invalid size.
 		// TODO(rsc): Merge with previous case (probably breaks toolstash -cmp).
 		case ir.OMAKESLICE, ir.OMAKESLICECOPY:
-			return n
+			return true
 		}
-		return nil
+		return false
 	})
-	return found != nil
 }
 
 // Rewrite