Adds the python script run-llvm2ice.py (was llvm2iceinsts.py) that
automatically handles conversion of LLVM source to a PEXE file,
and then runs llvm2ice on the corresponding PEXE file.

Also, defines three paths in tests, based on the executable chosen:

  %lc2i - Directly reads from LLVM source, and converts to Subzero.
  %l2i  - Parses a PEXE file into LLVM IR, and converts to Subzero.
  %p2i  - Parses a PEXE directly into Subzero.

Note that for all three executables, the same arguments can be used,
making it easy to change how the input is handled.

Also moves tests to use %p2i whenever possible.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3892
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/600043002