assertion failure: spirv-opt: "The header node is the entry block ?".

Running ./command in the attachment should reproduce the problem.

Brief summary: running spirv-opt (debug build) with arguments:

--eliminate-dead-branches --eliminate-dead-variables --redundancy-elimination --compact-ids --loop-unswitch

leads to the following error (numbers have been replaced with _ to avoid duplicates):

spirv-opt: source/opt/loop_descriptor.cpp:_: spvtools::opt::BasicBlock* spvtools::opt::Loop::FindLoopPreheader(spvtools::opt::DominatorAnalysis*): Assertion `loop_pred && "The header node is the entry block ?"' failed.

SPIRV-Tools v2018.7-dev v2018.6-94-geaa351a5

Found using [GraphicsFuzz](https://github.com/google/graphicsfuzz).

[1770.zip](https://github.com/KhronosGroup/SPIRV-Tools/files/2689812/1770.zip)

Start by running ./command from 1770.zip with the listed spirv-opt passes to reproduce the assertion. Then inspect source/opt/loop_descriptor.cpp, especially Loop::FindLoopPreheader and the failing header-node assumption. Done means the supplied reproducer no longer triggers the assertion.

Written by the indexing model from the issue text.