In the example from:
https://bugs.llvm.org/show_bug.cgi?id=38502
...we hit infinite looping/crashing because we have non-standard IR -
an instruction operand is used before defined.
This and other unusual constructs are allowed in unreachable blocks,
so avoid the problem by using DominatorTree to step around landmines.
Differential Revision: https://reviews.llvm.org/D67766
llvm-svn: 372339
13e71ce693