Storing PL_sv_yes over itself is as cheap as calling hv_exists(), so

Storing PL_sv_yes over itself is as cheap as calling hv_exists(), so
always store it, to save the (second) store call. SvIVx is more
expensive than SvIVX, so use the latter. Reorder the other call to
hv_store() to be immediately after the call to hv_exists(), to try to
avoid thrashing the CPU cache.

p4raw-id: //depot/perl@31009