[sancov] Pruning full dominator blocks from instrumentation.

Summary:
This is the first simple attempt to reduce number of coverage-
instrumented blocks.

If a basic block dominates all its successors, then its coverage
information is useless to us. Ingore such blocks if
santizer-coverage-prune-tree option is set.

Differential Revision: http://reviews.llvm.org/D17626

llvm-svn: 261949