[mlir][Arith] Fix up integer range inference for truncation

Attempting to apply the range analysis to real code revealed that
trunci wasn't correctly handling the case where truncation would
create wider ranges - for example, if we truncate [255, 257] : i16 to
i8, the result can be 255, 0, or 1, which isn't a contiguous range of
values.

The previous implementation would naively map this to [255, 1], which
would cause issues with unsigned ranges and unification.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D130501