fpu: Restrict parts_round_to_int_normal to target precision

fpu: Restrict parts_round_to_int_normal to target precision

Currently parts_round_to_int_normal() assumes that its input has just
been unpacked and therefore doesn't expect non-zero fraction bits past
target precision.

The upcoming DIVIDE TO INTEGER use cases needs it to support
calculations on intermediate values that utilize all fraction bits,
while at the same time restricting the result's precision to frac_size.

Delete the "All integral" check, because even though really large
values are always integer, their low fraction bits still need to be
truncated. For the same reason, make sure rnd_mask covers at least
fraction bits past target precision.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20260210214044.1174699-4-iii@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>