Avoid calling replace in normalizeSlashes when it would do nothing (#44100)

* Make normalizeSlashes a no-op there are no bad slashes

On Windows, there will probably be a negligible slowdown, iterating over
the pre-slash prefix of each unnormalized path (though we might come out
ahead if paths are normalized more than once).

On *nix, this saves work - 1.8s -> 0.4s in the project I'm
investigating.

* Reuse already-computed index

src/compiler/path.ts

@@ -452,6 +452,11 @@ namespace ts {
      * Normalize path separators, converting `\` into `/`.
      */
     export function normalizeSlashes(path: string): string {
+        const index = path.indexOf("\\");
+        if (index === -1) {
+            return path;
+        }
+        backslashRegExp.lastIndex = index; // prime regex with known position
         return path.replace(backslashRegExp, directorySeparator);
     }