go/ssa: remove unused func

This function was used by wellFormed, which was removed in CL 581835,
so it is of no use now. Remove it.

Updates golang/go#66783

Change-Id: I9b286238e2db93b364cef0dcdead397c4741c15e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/703035
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Kirill Kolyshkin <kolyshkin@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: kolyshkin

go/ssa/subst.go

@@ -567,76 +567,3 @@ func (subst *subster) signature(t *types.Signature) types.Type {
 	}
 	return t
 }
-
-// reaches returns true if a type t reaches any type t' s.t. c[t'] == true.
-// It updates c to cache results.
-//
-// reaches is currently only part of the wellFormed debug logic, and
-// in practice c is initially only type parameters. It is not currently
-// relied on in production.
-func reaches(t types.Type, c map[types.Type]bool) (res bool) {
-	if c, ok := c[t]; ok {
-		return c
-	}
-
-	// c is populated with temporary false entries as types are visited.
-	// This avoids repeat visits and break cycles.
-	c[t] = false
-	defer func() {
-		c[t] = res
-	}()
-
-	switch t := t.(type) {
-	case *types.TypeParam, *types.Basic:
-		return false
-	case *types.Array:
-		return reaches(t.Elem(), c)
-	case *types.Slice:
-		return reaches(t.Elem(), c)
-	case *types.Pointer:
-		return reaches(t.Elem(), c)
-	case *types.Tuple:
-		for i := 0; i < t.Len(); i++ {
-			if reaches(t.At(i).Type(), c) {
-				return true
-			}
-		}
-	case *types.Struct:
-		for i := 0; i < t.NumFields(); i++ {
-			if reaches(t.Field(i).Type(), c) {
-				return true
-			}
-		}
-	case *types.Map:
-		return reaches(t.Key(), c) || reaches(t.Elem(), c)
-	case *types.Chan:
-		return reaches(t.Elem(), c)
-	case *types.Signature:
-		if t.Recv() != nil && reaches(t.Recv().Type(), c) {
-			return true
-		}
-		return reaches(t.Params(), c) || reaches(t.Results(), c)
-	case *types.Union:
-		for i := 0; i < t.Len(); i++ {
-			if reaches(t.Term(i).Type(), c) {
-				return true
-			}
-		}
-	case *types.Interface:
-		for i := 0; i < t.NumEmbeddeds(); i++ {
-			if reaches(t.Embedded(i), c) {
-				return true
-			}
-		}
-		for i := 0; i < t.NumExplicitMethods(); i++ {
-			if reaches(t.ExplicitMethod(i).Type(), c) {
-				return true
-			}
-		}
-	case *types.Named, *types.Alias:
-		return reaches(t.Underlying(), c)
-	default:
-		panic("unreachable")
-	}
-	return false
-}