[sancov] Don't instrument blocks with no insertion point

This prevents crashes when attempting to instrument functions containing
C++ try.

Sanitizer coverage will still fail at runtime when an exception is
thrown through a sancov instrumented function, but that seems marginally
better than what we have now. The full solution is to color the blocks
in LLVM IR and only instrument blocks that have an unambiguous color,
using the appropriate token.

llvm-svn: 298662