intel/fs: Heap-allocate fs_visitors in brw_compile_fs().

intel/fs: Heap-allocate fs_visitors in brw_compile_fs().

This makes brw_compile_fs() look a bit more similar to
brw_compile_cs().  It saves us three v*_shader_stats local variables,
and will save us additional triplicated declarations as we start
tracking IR performance analysis results.

The triplicated cfg pointers are left around because they're set to
NULL to mark specific dispatch modes as disabled (e.g. in order to
enforce hardware restrictions).  Doing the same thing with the visitor
pointers would cause data leaks.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>