lgc/PatchInOutImportExport: assert that vertex fetch is handled elsewhere
lgc/PatchInOutImportExport: assert that vertex fetch is handled elsewhere

This code leads to a switch statement in which vertex shaders would fall
into llvm_unreachable, so this is just a slight cleanup and no functional
change.

v2:
- formatting fix

GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc,GPUOpen-Drivers/llpc