Another faulty padrange assumption

Commit 7601007 was not sufficient.  There are two places where the
padrange optimisation tries to combine multiple padranges.

When a padrange op is created in rpeep, the code first checks whether
the previous op is already a padrange, so the two can be combined, as
in this case:

    my ($a,$b,$c);
    my ($d,$e,$f);

Then the code checks whether it can swallow up any singletons follow-
ing it, optimising cases like this:

    my ($v,$w,$x);
    my $y;

Commit 7601007 fixed the latter, which was assuming that $x and $y
would have contiguous pad offsets.

This commit fixes the former code, which assumed $c and $d would have
contiguous offsets.

This was causing assertion failures or crashes for Devel::CallParser
0.001 (0.002 works around it), because Devel::CallParser creates new
pad entries when the second ‘my’ keyword is encountered, causing the
pad offsets not to be contiguous.