split pp_predec() from pp_preinc() and improve

pp_preinc() handles both ++$x and --$x (and the integer variants
pp_i_preinc/dec). Split it into two separate functions, as handling
both inc and dec in the same function requires 3 extra conditionals.

At the same time make the code more efficient.

As currently written it:
1) checked for "bad" SVs (such as read-only) and croaked;
2) checked for a IOK-only SV and directly incremented the IVX slot;
3) else called out to sv_inc() to handle the more complex cases.

This commit combines the checks in (1) and (2) into one single big
check of flags, and anything "bad" simply skips the IOK-only code
and calls sv_dec(), which can do its own checking of read-only etc
and croak if necessary. Porting/bench.pl shows the following raw numbers
for ++$x ($x lexical and holding an integer):

         before    after
       -------- --------
    Ir     77.0     56.0
    Dr     30.0     24.0
    Dw     10.0     10.0
  COND     12.0      9.0
   IND      2.0      2.0

COND_m     -0.1      0.0
 IND_m      2.0      2.0

Even having split the function into two, the combined size of the two new
functions is smaller than the single previous function.

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