refactor Perl_cv_undef_flags

On VC 2003 32bits size of this function decreased from 0x321 bytes of
machine code to 0x2d8.

cv.h:
- partially reorder Cv* macros to match XPVCV member order
- create CvDEPTHunsafe which never uses SV head except for SvANY ptr since
  Perl_cv_undef_flags uses a fake SV head

pad.c
- remove var slabbed, frees a C auto/non-vol register, there are only 2
  uses, on CvSTART branch, CvFLAGS is reused frm CvISXSUB test by optimizer
- use a CV struct to fake a CV, to avoid rereading SvANY ptr after each
  func call, CVs can't be upgraded or have their bodies realloced
- dont write NULL to CvFILE if CvFILE is NULL, also move NULL assignment
  so CPU address generation can be reused by compiler
- refactor CvROOT/CvSTART/CvXSUB freeing conditionals to simplify code and
  dont check CvISXSUB twice
- CvDEPTH requires a real CV*/CV*, since it checks the SV head with an
  assert, use CvDEPTHunsafe instead, and inline the assert using the real
  CV*. Also move runtime, non-debug "SvTYPE(cv) == SVt_PVCV" check to
  debug  builds per ML post
  "about FC commit "CV-based slab allocation for ops""
- Perl_croak->Perl_croak_nocontext, remove push arg my_perl instruction
- refactor CvPADLIST freeing for provision for future XSUB sub usage of
  CvPADLIST in a union
- in CvOUTSIDE freeing, move NULL assignment so CPU address generation can
  be reused by compiler