fix corner cases in switch and undefined (#1762)

- fix side effects in switch condition for singular blocks
- fix `undefined` confusion with local variable
- gate `OPT(AST_Switch)` with `switches`

fixes #1758
fixes #1759

test/compress/issue-1750.js

@@ -2,6 +2,7 @@ case_1: {
     options = {
         dead_code: true,
         evaluate: true,
+        switches: true,
     }
     input: {
         var a = 0, b = 1;