Kick out of normalizePath if there's nothing to do

...using `relativePathSegmentRegExp`.

Bonus: use a regex to handle "/./" to avoid splitting and joining in a
common case.

When building the compiler, for example, it looks like ~95% of arguments
to `normalizePath` do not require any normalization.

Author: amcasey

src/compiler/path.ts

@@ -547,6 +547,10 @@ namespace ts {
 
     export function normalizePath(path: string): string {
         path = normalizeSlashes(path);
+        path = path.replace(/\/\.\//g, "/");
+        if (!relativePathSegmentRegExp.test(path)) {
+            return path;
+        }
         const normalized = getPathFromPathComponents(reducePathComponents(getPathComponents(path)));
         return normalized && hasTrailingDirectorySeparator(path) ? ensureTrailingDirectorySeparator(normalized) : normalized;
     }
@@ -658,7 +662,7 @@ namespace ts {
     //// Path Comparisons
 
     // check path for these segments: '', '.'. '..'
-    const relativePathSegmentRegExp = /(^|\/)\.{0,2}($|\/)/;
+    const relativePathSegmentRegExp = /(?:^|\/)\.\.?(?:$|\/)|\/\//;
 
     function comparePathsWorker(a: string, b: string, componentComparer: (a: string, b: string) => Comparison) {
         if (a === b) return Comparison.EqualTo;