make my (...) = @_ non-OPpASSIGN_COMMON_RC1

Technically in

    my ($scalar,...) = @_

due to closure/goto tricks, its possible for $scalar to appear on both
the LHS and RHS, so we currently set the OPpASSIGN_COMMON_RC1 flag.

However, this imposes extra overhead; for example 5% extra instruction
reads and 11% extra conditional branches for

    my ($x,$y,$z) = @_;

Given what an important construct this is, disable this flag in the
specific case of of only my's on the LHS and only @_ on the RHS.
It's technically incorrect, but its the same behaviour we've always had
(it was only the previous commit which made it safe but slower).

We still set the OPpASSIGN_COMMON_AGG flag for

    my ($...,@a) = @_

since in the normal case this only adds the small additional runtime
overhead of checking that @a is already empty.

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