fix: do not inline regexp literal when declaration in loop

packages/babel-plugin-minify-dead-code-elimination/__tests__/fixtures/issue-981/actual.js

@@ -0,0 +1,5 @@
+function f(query) {
+  var r = /./g,
+    part;
+  while (part = r.exec(query));
+}

packages/babel-plugin-minify-dead-code-elimination/__tests__/fixtures/issue-981/expected.js

@@ -0,0 +1,6 @@
+function f(query) {
+  var r = /./g,
+      part;
+
+  while (part = r.exec(query));
+}

packages/babel-plugin-minify-dead-code-elimination/src/index.js

@@ -421,7 +421,8 @@ module.exports = ({ types: t, traverse }) => {
               const isObj = n =>
                 t.isFunction(n) ||
                 t.isObjectExpression(n) ||
-                t.isArrayExpression(n);
+                t.isArrayExpression(n) ||
+                t.isRegExpLiteral(n);
 
               const isReplacementObj =
                 isObj(replacement) || some(replacement, isObj);