llvm/test/BugPoint/compile-custom.ll.py: Make it py3-compatible. [PR19112]

FIXME: Get rid of invoking this.
I guess it wouldn't run on win32 due to lacking of shell support.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203740 91177308-0d34-0410-b5e6-96231b3b80d8