Micro-optimize PATH.basename. NFC

[sbc100]

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%)