Performance tweaks

Better performance for %cmp/s and the vvp_fun_part_sa node.
Optimizations for %pushi/vec4 and %load/vec4

- Have %pushi/vec4 handle some special cases optimally.
- Eliminate some duplicated method calls in %load/vec4.
- Optimize the vvp_vector4_t::copy_from_ method by inlining
some parts.