Makes IceTranslator.ExitStatus a boolean (rather than int), and changes
code to check flag when done. Fixes bug introduced in
https://codereview.chromium.org/387023002.

Also cleans up the (Ice) Converter class to handle globals processing,
rathe than doing it in llvm2ice.cpp.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3894
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/387023002