treewide: kill problematic "$h->{k} //= do {" assignments

treewide: kill problematic "$h->{k} //= do {" assignments

As stated in the previous change, conditional hash assignments
which trigger other hash assignments seem problematic, at times.
So replace:

	$h->{k} //= do { $h->{x} = ...; $val };

	$h->{k} // do {
		$h->{x} = ...;
		$hk->{k} = $val
	};

"||=" is affected the same way, and some instances of "||=" are
replaced with "//=" or "// do {", now.