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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176668 91177308-0d34-0410-b5e6-96231b3b80d8
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