layers: Simplify generated param validation code
layers: Simplify generated param validation code

- Remove unnecessary conditional checks for input vs. output parameters
  from the generated C++ code.
- Split some generator code into smaller functions.

Change-Id: I32e47d417ab884e5cc0fc7af40cb5657b39c176d

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools