Attempting to optimize using --target-env=vulkan1.1 -O fails with this message:
shaderc: internal error: compilation succeeded but failed to optimize: Invalid SPIR-V binary version 1.3 for target environment SPIR-V 1.0 (under Vulkan 1.0 semantics).
I have attached a patch. It gives you an idea of what needs to be done. It is not super great but gets the job done.
vulkan11opt.patch.txt