make OP_REGCRESET only for taint handling

make OP_REGCRESET only for taint handling

The OP_REGCRESET op, which is sometimes prepended to the chain of ops
leading to OP_REGCOMP, currently serves two purposes; first to reset the
taint flag, and second to initialise PL_reginterp_cnt. The second purpose
is no longer needed, and the first has a bug, in that the op isn't
prepended when "use re 'eval'" is in scope.

Fix this by prepending the op solely when PL_tainting is in effect.
This also makes run-time regexes slightly more efficient in the
non-tainting case.