Fix a latent issue when we try to pack FS input
Fix a latent issue when we try to pack FS input

We shouldn't use shader stage and argument count to check if the FS
input call is interpolant call.

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