[llc] Remove exit-on-error flag (PR27759)

This flag was introduced in r269655 with the new diagnostic handler for llc. Its
purpose was to keep the old behavior for some of the tests that didn't recover
well after an error. Those tests have been fixed, so now it's safe to remove the
flag entirely.

Fixes PR27759.

Differential Revision: http://reviews.llvm.org/D21368

llvm-svn: 273554