[Float2Int] avoid crashing on unreachable code (PR38502)

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