lgc/VertexFetch: some minor code cleanups
lgc/VertexFetch: some minor code cleanups

Merge two loops over the vertex fetches, and re-use information that was
already extracted.

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