ac: fix broken elimination of duplicated VS exports

ac: fix broken elimination of duplicated VS exports

The renumbering code didn't take into account that multiple VS exports
can have the same PARAM index. This also significantly simplifies
the renumbering. Thankfully, we have piglits for this:

    spec@arb_gpu_shader5@arb_gpu_shader5-interpolateatcentroid-packing
    spec@glsl-1.50@execution@interface-blocks-complex-vs-fs

Reported by Michel Dänzer.

Fixes: b08715499e61 ("ac: eliminate duplicated VS exports")
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>