Don’t allow OPpTARGET_MY with integer negation

Don’t allow OPpTARGET_MY with integer negation

$ ./perl -Ilib -le 'use integer; my $a = "fake"; $a = -$a; print "[$a]"'
[--]

As of 1c2b3fd6f1, negation under ‘use integer’ can do string negation,
which modifies the return value before reading the argument.  So, like
regular non-integer negation, it must forego this optimisation.