Loop peeling creates OpPhi on pointer.

[1018.zip](https://github.com/KhronosGroup/SPIRV-Tools/files/2689794/1018.zip)
Running ./command in the attachment should reproduce the problem.

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

--redundancy-elimination --loop-peeling --eliminate-common-uniform

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

spirv-opt: source/opt/common_uniform_elim_pass.cpp:_: spvtools::opt::Instruction* spvtools::opt::CommonUniformElimPass::GetPtr(spvtools::opt::Instruction*, uint__t*): Assertion `objInst->opcode() == SpvOpCopyObject' failed.

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

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

[1018.zip](https://github.com/KhronosGroup/SPIRV-Tools/files/2689798/1018.zip)
[0484](https://github.com/KhronosGroup/SPIRV-Tools/files/2689788/0484.zip)

Start by running the ./command from the attached 1018.zip reproducer with spirv-opt using --redundancy-elimination, --loop-peeling, and --eliminate-common-uniform. Read source/opt/common_uniform_elim_pass.cpp around CommonUniformElimPass::GetPtr and investigate the assertion that the object instruction must be OpCopyObject. Done means the reproducer no longer triggers the assertion.

Written by the indexing model from the issue text.