[llc] Remove exit-on-error flag from MIR tests (PR27770)

This is made possible by removing an assert in llc that assumed
MIRParser::parseLLVMModule would exit on error. MIRParser's documentation states
that it returns null if a parsing error occurs, so there's no reason to assert.
We can instead just fall through to where the check for a module is performed
and exit if it is null.

This commit is part of the clean-up after r269655.

Fixes PR27770

Differential Revision: http://reviews.llvm.org/D20371

llvm-svn: 272254