gas: special-case division / modulo by ±1

Dividing the largest possible negative value by -1 generally is UB, for
the result not being representable at least in commonly used binary
notation. This UB on x86, for example, is a Floating Point Exception on
Linux, i.e. resulting in an internal error (albeit only when
sizeof(valueT) == sizeof(void *); the library routine otherwise involved
apparently deals with the inputs quite okay).

Leave original values unaltered for division by 1; this may matter down
the road, in case we start including X_unsigned and X_extrabit in
arithmetic. For the same reason treat modulo by 1 the same as modulo by
-1.

The quad and octa tests have more relaxed expecations than intended, for
X_unsigned and X_extrabit not being taken into account [yet]. The upper
halves can wrongly end up as all ones (for .octa, when !BFD64, even the
upper three quarters). Yet it makes little sense to address this just
for div/mod by ±1. quad-div2 is yet more special, to cover for most
32-bit targets being unable to deal with forward-ref expressions in
.quad even when BFD64; even ones being able to (like x86) then still
don't get the values right.

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