fix switch branch elimination (#1752)

Merge unreachable case body with previous fallthrough case

fixes #1750

test/compress/issue-1750.js

@@ -0,0 +1,25 @@
+case_1: {
+    options = {
+        dead_code: true,
+        evaluate: true,
+    }
+    input: {
+        var a = 0, b = 1;
+        switch (true) {
+          case a, true:
+          default:
+            b = 2;
+          case true:
+        }
+        console.log(a, b);
+    }
+    expect: {
+        var a = 0, b = 1;
+        switch (true) {
+          case a, true:
+            b = 2;
+        }
+        console.log(a, b);
+    }
+    expect_stdout: "0 2"
+}