Define OP_HELEMEXISTSOR, a handy LOGOP shortcut for HELEM existence tests

This op is constructed using an OP_HELEM as the op_first and any scalar
expression as the op_other.

It is roughly equivalent to the following perl code:

  exists $hv{$key} ? $hv{$key} : OTHER

except that the HV and the KEY expression are evaluated only once, and
only one hv_* function is invoked to both test and obtain the value. It
is therefore smaller and more efficient.

Likewise, adding the OPpHELEMEXISTSOR_DELETE flag turns it into the
equivalent of

  exists $hv{$key} ? delete $hv{$key} : OTHER

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