RT #67962: $1 treated as tainted in untainted match

RT #67962: $1 treated as tainted in untainted match

Fix the issue in the following:

    use re 'taint';
    $tainted =~ /(...)/;
    # $1 now correctly tainted
    $untainted =~ s/(...)/$1/;
    # $untainted now incorrectly tainted

The problem stems from when $1 is updated.

pp_substcont, which is called after the replacement expression has been
evaluated, checks the returned expression for taintedness, and if so,
taints the variable being substituted. For a substitution like
s/(...)/x$1/ this works fine: the expression "x".$1 causes $1's get magic
to be called, which sets $1 based on the recent match, and is marked as
not tainted.  Thus the returned expression is untainted. In the variant
s/(...)/$1/, the returned value on the stack is $1 itself, and its get
magic hasn't been called yet. So it still has the tainted flag from the
previous pattern.

The solution is to mg_get the returned expression *before* testing for
taintedness.