gopls/internal/analysis/modernize: rangeint: fix yet another bug

ASSIGN and DEFINE are not the only kinds of AssignStmt.

+ test

Change-Id: I81b5122b0ac0db9be5178b6685c00212f3c4c469
Reviewed-on: https://go-review.googlesource.com/c/tools/+/660695
Reviewed-by: Jonathan Amsterdam <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: adonovan

gopls/internal/analysis/modernize/rangeint.go

@@ -263,8 +263,8 @@ func isScalarLvalue(info *types.Info, curId cursor.Cursor) bool {
 	switch ek {
 	case edge.AssignStmt_Lhs:
 		assign := cur.Parent().Node().(*ast.AssignStmt)
-		if assign.Tok == token.ASSIGN {
-			return true // i = j
+		if assign.Tok != token.DEFINE {
+			return true // i = j or i += j
 		}
 		id := curId.Node().(*ast.Ident)
 		if v, ok := info.Defs[id]; ok && v.Pos() != id.Pos() {

gopls/internal/analysis/modernize/testdata/src/rangeint/rangeint.go

@@ -132,6 +132,9 @@ func _(i int, s struct{ i int }, slice []int) {
 			s = s[1:]
 		}
 	}
+	for i := 0; i < len(slice); i++ { // nope: i is incremented within loop
+		i += 1
+	}
 }
 
 var Global int

gopls/internal/analysis/modernize/testdata/src/rangeint/rangeint.go.golden

@@ -132,6 +132,9 @@ func _(i int, s struct{ i int }, slice []int) {
 			s = s[1:]
 		}
 	}
+	for i := 0; i < len(slice); i++ { // nope: i is incremented within loop
+		i += 1
+	}
 }
 
 var Global int