[sancov] do not instrument nodes that are full pre-dominators

Summary:
Without tree pruning clang has 2,667,552 points.
Wiht only dominators pruning: 1,515,586.
With both dominators & predominators pruning: 1,340,534.

Differential Revision: http://reviews.llvm.org/D17671

llvm-svn: 262103