spirv-val should report SPIR-V file as invalid due to jump from loop to non-merge block

[SPIR-V binary in this archive illustrates the problem](https://github.com/KhronosGroup/SPIRV-Tools/files/2541463/spirv-val-bug.zip).

Quoting @greg-lunarg from #2018:

> Pretty sure that [bad].spv is invalid: it contains a branch in a loop to a block outside the loop (%46) that is not the loop's merge block.

spirv-val should ideally catch this; current version does not.

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

Start by reproducing the report with the SPIR-V binary in the linked archive and the spirv-val entry point. Trace validation of the branch from the loop to block %46, using issue #2018 for context. Done means spirv-val rejects the invalid module and includes a regression test for this case.

Written by the indexing model from the issue text.