lit: Util.which(): Use os.path.isfile() instead of os.path.exists(), or it hits to the directory.
lit: Util.which(): Use os.path.isfile() instead of os.path.exists(), or it hits to the directory.

For example, which('loop-convert') returns 'loop-convert' when the directory 'loop-convert' exists.

git-svn-id: a4a6f32337ebd29ad4763b423022f00f68d1c7b7@163469 91177308-0d34-0410-b5e6-96231b3b80d8

lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx