PVFMs don't need CvDEPTH, and PVCVs don't use SvIVX, so moving

PVFMs don't need CvDEPTH, and PVCVs don't use SvIVX, so moving
xcv_depth into the IV union saves 4(ish) bytes per CV and format.
"ish" because it was a long, but has been changed to I32 (along with
the corresponding field in struct block_sub) so as not to enlarge the
IV union on platforms where sizeof(long) > sizeof(IV), or struct
block_sub where sizeof(long) > sizeof(I32)

p4raw-id: //depot/perl@27247