gas: drop scrubber state -2

Instead re-use code handling LEX_IS_TWOCHAR_COMMENT_1ST, thus ensuring
that we wouldn't get bogus state transitions: For example, when we're in
states 0 or 1, a comment should be no different from whitespace
encountered in those states. Plus for e.g. x86 this results in such
comments now truly being converted to a blank, as mandated by
documentation. Both aspects apparently were a result of blindly (and
wrongly) moving to state 3 _before_ consuming the "ungot" blank.

Also amend a related comment elsewhere.

In the new testcase the .irp is to make visible in the listing all the
whitespace that the scrubber inserts / leaves in place.