refactor pp_stringify and pp_and

refactor pp_stringify and pp_and

pp_stringify:

First, move all the SP/stack operations to before the first call in
pp_stringify (sv_copypv), this allows SP to never be saved across any calls.
SETTARG calls set magic, so that still has to be done. SETs does not ++/--
SP, so no need to do a PUTBACK. PL_op is read twice, once for TARG, once
for NORMAL. No point in caching it through the sv_copypv call since its not
worth saving a non-vol to C stack (x86) just to avoid 2 mem acesses. This
opcode now only has 2 vars saved across sv_copypv, my_perl and TARG.

pp_stringify dropped from 0x3F to 0x3C for me after these changes, 32 bit
x86 VC 2003.

pp_and:

Do the PERL_ASYNC_CHECK before anything else, calcing SP before
PERL_ASYNC_CHECK would mean saving it across the potential call. SvTRUE
macro checks its param for NULL. This SV * came off the Perl stack, it will
not be null since pp_and is not a core sub (see S_maybe_add_coresub) and
will not be called from pp_coreargs (pp_coreargs places (SV *)NULLs on
Perl stack). So create a SvTRUE_NN macro that does not check for NULL, this
saves a branch. Since the Perl stack is only touched twice, don't create
a local SP. See comment in code. This saves another variable to not be
saved across the sv_2bool_flags and removed PUTBACK instructions. PL_op is
only read once or twice (compiler choice) right before the 2 returns.
There is only 1 variable saved across any calls in pp_and now, and that is
my_perl. dSP also was not use to prevent an accidental use of a ++/-- SP
macro in pp_and in the future since there is no PUTBACK now. I guess for a
SPARC with register windowing and no direct memory instructions, accessing
PL_stack_sp twice will take more instructions 2 reads and a write to
PL_stack_sp vs 1 read and 1 write to PL_stack_sp with a dSP design. On
SPARC, with a register window, cross call regs have no cost per reg cost
aslong as you dont exceed their max number.

For me, pp_and went from 0x112 to 0xFF after this commit. Prior to dSP
removal, it was down to 0x109. SvTRUE_NN was named after SvREFCNT_inc_NN.
There remains an inefficiency in pp_and on non-GCC compilers due to
SvPVXtrue using PL_Xpv which will be dealt with in another commit.