intel/vec4: Make implied_mrf_writes() a vec4_instruction method

Same as commit c20dc9b8363b (intel/fs: Make implied_mrf_writes() an
fs_inst method.)

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4093>
4 files changed