Fix for PR16570: when comparing two function pointers, discard qualifiers when
comparing non-reference function parameters.  The qualifiers don't matter for
comparisons.

This is a re-commit of r187769, which was accidentially reverted in r187770,
with a simplification at the suggestion of Eli Friedman.
DataFlowSanitizer: Remove unreachable BBs so IR continues to verify
under the args ABI.

Differential Revision: http://llvm-reviews.chandlerc.com/D1316