pan/bi: Imply round mode most of the time

pan/bi: Imply round mode most of the time

Much less noisy, and provides a path to further improvements. There is a slight
behaviour change: int-to-float conversions now use RTE instead of RTZ. For
32-bit opcodes, this affects conversions of integers with magnitude greater than
2^23 by at most 1 ulp. As this behaviour is unspecified in GLSL, this change is
believed to be acceptable.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15187>