Inline SetupVertexDecoder() in SubmitPrim().

Everything SubmitPrim() calls is expensive...
Avoid some debug stats when disabled.

This function can get called a lot sometimes, and an if helps on ARM a
little.
Simplify UpdatePC() a bit more.
Small perf improvements in DecodeVertsStep().
Merge pull request #5808 from unknownbrackets/perf

More small performance tweaks based on ARM profiling