Fix the lowering of `arith.truncf : f32 to bf16`. (#83180)

This lowering was not correctly handling the case where saturation of
the mantissa results in an increase of the exponent value. The new code
borrows, with credit, the idea from
https://github.com/pytorch/pytorch/blob/e1502c0cdbfd17548c612f25d5a65b1e4b86224d/c10/util/BFloat16.h#L60-L79
and adds comments to explain the magic trick going on here and why it's
correct. Hat tip to its original author, whom I believe to be
@Maratyszcza.

A testcase was also requiring a tie to be broken upwards in a case where
"to nearest-even" required going downward. The fact that it used to pass
suggests that there was another bug in the old code.