[MLIR] Flatten fused locations when merging constants. (#75218)

[PR 74670](https://github.com/llvm/llvm-project/pull/74670) added
support for merging locations at constant folding time. We have
discovered that in some cases, the number of locations grows so big as
to cause a compilation process to OOM. In that case, many of the
locations end up appearing several times in nested fused locations.

We add here a helper that always flattens fused locations in order to
eliminate duplicates in the case of nested fused locations.