Remove some dead code in tgt-vvp.
Fix botched implementations of vec4 based shift.
vec4 based calls to simple real system functions.
vec4 versions of the %force instructions.
WRap up vec4 support for system function calls.
Cleanup vec4 handling of suppressed store/assign due to bad offset/index math.
Wrap up vpi access to vec4 stack items.
Fix possible overflow in calculation of canonical indices.
Fix %store/vec4 overlap of vectors written with extreme indices.