Rename isEarlierInSameTrace to isUsefulDominator.

In very rare cases caused by irreducible control flow, the dominating
block can have the same trace head without actually being part of the
trace.

As long as such a dominator still has valid instruction depths, it is OK
to use it for computing instruction depths.

Rename the function to avoid lying, and add a check that instruction
depths are computed for the dominator.

llvm-svn: 176668