tgsi/scan: handle holes between VS inputs, assert-fail in other cases

tgsi/scan: handle holes between VS inputs, assert-fail in other cases

"st/mesa: overhaul vertex setup for clearing, glDrawPixels, glBitmap"
added a vertex shader declaring IN[0] and IN[2], but not IN[1].

Drivers relying on tgsi_shader_info can't handle holes in declarations,
because tgsi_shader_info doesn't track that.

This is just a quick workaround meant for stable that will work for vertex
shaders.

This fixes radeonsi DrawPixels and CopyPixels crashes.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Brian Paul <brianp@vmware.com>