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: https://llvm.org/svn/llvm-project/llvm/trunk@163469 91177308-0d34-0410-b5e6-96231b3b80d8

utils/lit/lit/Util.py

@@ -56,7 +56,7 @@ def which(command, paths = None):
         paths = os.environ.get('PATH','')
 
     # Check for absolute match first.
-    if os.path.exists(command):
+    if os.path.isfile(command):
         return command
 
     # Would be nice if Python had a lib function for this.