intel/fs: Be more conservative in split_virtual_grfs

intel/fs: Be more conservative in split_virtual_grfs

Instead of modifying every single instruction, keep track of which VGRFs
are actually split in a bit-set, and only modify the instructions that
actually touch split regs.

This cuts the runtime of the following Vulkan CTS on my SKL box by 45%
from 3:21 to 1:51:  dEQP-VK.ssbo.phys.layout.random.16bit.scalar.13

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13734>