Allow multideref for $x[($lex)]

The parens don’t need to disable the optimisation.

(The OPpLVAL_INTRO flag [indicating my or state] has no run-time
effect for state vars, so we could ignore it and optimise, but it
would make deparsing inordinately complex, and no-one would write
$x[state $y] anyway.)
Fix assertion failure with $x[($_)]
Fix assertion failure with $x[CONST->[0]]
Allow multideref for $x[($pkg)]

The parens don’t need to disable the optimisation.
Allow multideref for $x[CONST->$*]
dump.c:unop_aux_stringify: Use newSVpvn_flags

newSVpvn_flags with SVs_TEMP takes less machine code than
sv_2mortal(newSVpv()).
dump.c: Suppress extra sigils in multideref dump

This:

  my $z; my @y; $y[$z]

included

  <+> multideref($@y[$$z]) sK ->6

in its -MO=Concise output.
Skip multideref for PL_check[OP_HELEM] tinkering

We already do it when PL_check[OP_AELEM] is modified.  Do it
also for PL_check[OP_HELEM].  This will make it easier to get
Data::Alias working.