Use new regnodes for /[[:ascii:]]/

Use new regnodes for /[[:ascii:]]/

Prior to this commit, the ASCII Posix class was treated as any other
Posix class under /a.  It turns out that by making separate nodes for it
(and its complement), the performance gains can be astronomical on ASCII
platforms.  This is mainly due to the fact that scanning can be done
word-at-a-time, but also because various conditionals can be skipped.

Below are some measurements from Porting/bench.pl on a 64-bit Linux g++
-O2 system.The numbers are for very long strings, as otherwise, the
delta due solely to this change is masked by the overhead around pattern
matching in general.  These numbers are for finding an ASCII character
at the end of a very long string of non-ASCII ones.

All 1-byte (non-utf8) characters improvement ratio
    Ir      1990.8
    Dr      2995.4
    Dw      17296.7
  COND      786.0

All 2-byte characters improvement ratio
    Ir      242.2
    Dr      232.9
    Dw      17237.8
  COND      100.0

The numbers for three and more bytes per character are essentially the
same as for two bytes.

As an example, the Dw for strings consisting entirely of 2-byte
characters (before the terminal ASCII one) now take 584 instructions,
and previously 100,582

The gain would be less for a 32-bit system.

bench.pl returns other measurements which I omitted above, because they
either have unchanged performance or involve a trivial number of total
instructions.