Add NQP version of split_string_constant

Also switch Perl 5 and Perl 6 variants to while loops to match NQP,
and add regex tag to split_string_regex.

Author: Geoffrey Broadwell

microbenchmarks.pl

@@ -293,16 +293,16 @@
  },
  {
   name  => 'split_string_constant',
-  tags  => [qw( for string )],
+  tags  => [qw( while string )],
   scale => 1 << 7,
   work  => sub { $_[0] * $_[0] },
-  perl5 => 'my $s = join ", " => 1 .. SCALE; my @s = split ", " => $s for 1 .. SCALE',
-  perl6 => 'my $s = (1 .. SCALE).join: ", "; my @s = $s.split(", ") for 1 .. SCALE',
-  nqp   => undef,
+  perl5 => 'my $s = join ", " => 1 .. SCALE; my $i; my @s = split ", " => $s while ++$i <= SCALE',
+  perl6 => 'my $s = (1 .. SCALE).join: ", "; my $i; my @s = $s.split(", ") while ++$i <= SCALE',
+  nqp   => 'my @i; my $i := 0; while ($i := $i + 1) <= SCALE { nqp::push(@i, ~$i); }; my $s := nqp::join(", ", @i); $i := 0; while ($i := $i + 1) <= SCALE { my @s := nqp::split(", ", $s) }',
  },
  {
   name  => 'split_string_regex',
-  tags  => [qw( for string )],
+  tags  => [qw( for string regex )],
   scale => 1 << 7,
   work  => sub { $_[0] * $_[0] },
   perl5 => 'my $s = join ", " => 1 .. SCALE; my @s = split /\s*,\s*/ => $s for 1 .. SCALE',