Micro-optimize PATH.basename. NFC (emscripten-core#23198)

The following (18) test expectation files were updated by
running the tests with `--rebaseline`:
```                                                                      
other/codesize/test_codesize_cxx_ctors1.gzsize: 8507 => 8482 [-25 bytes / -0.29%]
other/codesize/test_codesize_cxx_ctors1.jssize: 20871 => 20818 [-53 bytes / -0.25%]
other/codesize/test_codesize_cxx_ctors2.gzsize: 8491 => 8466 [-25 bytes / -0.29%]
other/codesize/test_codesize_cxx_ctors2.jssize: 20839 => 20786 [-53 bytes / -0.25%]
other/codesize/test_codesize_cxx_except.gzsize: 9537 => 9516 [-21 bytes / -0.22%]
other/codesize/test_codesize_cxx_except.jssize: 24716 => 24663 [-53 bytes / -0.21%]
other/codesize/test_codesize_cxx_except_wasm.gzsize: 8467 => 8443 [-24 bytes / -0.28%]
other/codesize/test_codesize_cxx_except_wasm.jssize: 20765 => 20711 [-54 bytes / -0.26%]
other/codesize/test_codesize_cxx_except_wasm_exnref.gzsize: 8467 => 8443 [-24 bytes / -0.28%]
other/codesize/test_codesize_cxx_except_wasm_exnref.jssize: 20765 => 20711 [-54 bytes / -0.26%]
other/codesize/test_codesize_cxx_lto.gzsize: 8406 => 8389 [-17 bytes / -0.20%]
other/codesize/test_codesize_cxx_lto.jssize: 20455 => 20402 [-53 bytes / -0.26%]
other/codesize/test_codesize_cxx_mangle.gzsize: 9541 => 9523 [-18 bytes / -0.19%]
other/codesize/test_codesize_cxx_mangle.jssize: 24716 => 24663 [-53 bytes / -0.21%]
other/codesize/test_codesize_cxx_noexcept.gzsize: 8507 => 8482 [-25 bytes / -0.29%]
other/codesize/test_codesize_cxx_noexcept.jssize: 20871 => 20818 [-53 bytes / -0.25%]
other/codesize/test_codesize_files_js_fs.gzsize: 7646 => 7621 [-25 bytes / -0.33%]
other/codesize/test_codesize_files_js_fs.jssize: 18784 => 18731 [-53 bytes / -0.28%]
                                                                         
Average change: -0.26% (-0.33% - -0.19%)                                 
```

Author: sbc100

src/library_path.js

@@ -63,14 +63,9 @@ addToLibrary({
       }
       return root + dir;
     },
-    basename: (path) => {
-      // EMSCRIPTEN return '/' for '/', not an empty string
-      if (path === '/') return '/';
-      path = path.replace(/\/+$/g, "");
-      var lastSlash = path.lastIndexOf('/');
-      if (lastSlash === -1) return path;
-      return path.substr(lastSlash+1);
-    },
+    // This differs from node's path.basename in that it returns '/' for '/'
+    // rather than the empty string.
+    basename: (path) => path && path.match(/([^\/]+|\/)\/*$/)[1],
     join: (...paths) => PATH.normalize(paths.join('/')),
     join2: (l, r) => PATH.normalize(l + '/' + r),
   },

test/other/codesize/test_codesize_cxx_ctors1.gzsize

@@ -1 +1 @@
-8507
+8482

test/other/codesize/test_codesize_cxx_ctors1.jssize

@@ -1 +1 @@
-20871
+20818

test/other/codesize/test_codesize_cxx_ctors2.gzsize

@@ -1 +1 @@
-8491
+8466

test/other/codesize/test_codesize_cxx_ctors2.jssize

@@ -1 +1 @@
-20839
+20786

test/other/codesize/test_codesize_cxx_except.gzsize

@@ -1 +1 @@
-9537
+9516

test/other/codesize/test_codesize_cxx_except.jssize

@@ -1 +1 @@
-24716
+24663

test/other/codesize/test_codesize_cxx_except_wasm.gzsize

@@ -1 +1 @@
-8467
+8443

test/other/codesize/test_codesize_cxx_except_wasm.jssize

@@ -1 +1 @@
-20765
+20711

test/other/codesize/test_codesize_cxx_except_wasm_exnref.gzsize

@@ -1 +1 @@
-8467
+8443