Avoid running the validator as part of the optimization pass.

The spvtools::Optimizer::Run method glslang is using constructs a default set
of spvtools::OptimizerOptions. This default set of options instructs the
validator to run. That is not quite correct since glslang will invoke the
validator _explicitly_ after the optimization pass.

Change-Id: I30f458304c6e7f81e89fc4ebd25eabbbd8348063