vc4: Add shader-db dumping of NIR instruction count.

I was previously using temporary disables of VC4 optimization to show the
benefits of improved NIR optimization, but this can get me quick and dirty
numbers for NIR-only improvements without having to add hacks to disable
VC4's code (disabling of which might hide ways that the NIR changes would
hurt actual VC4 codegen).
1 file changed