Revert "[LSV] Merge contiguous chains across scalar types" (#170381)

Reverts llvm/llvm-project#154069. I pointed out a number of issues
post-merge, most importantly examples of miscompiles:
https://github.com/llvm/llvm-project/pull/154069#issuecomment-3603854626.

While the motivation of the change is clear, I think the implementation
approach is flawed. It seems like the goal is to allow elements like
`load <2xi16>` and `load i32` to be vectorized together despite the
current algorithm not grouping them into the same equivalence classes. I
personally think that if we want to attempt this it should be a more
wholistic approach, maybe even redefining the concept of an equivalence
class. This current solution seems like it would be really hard to do
bug-free, and even if the bugs were not present, it is only able to
merge chains that happen to be adjacent to each other after
`splitChainByContiguity`, which seems like it is leaving things up to
chance whether this optimization kicks in. But we can discuss more in
the re-land. Maybe the broader approach I'm proposing is too difficult,
and a narrow optimization is worthwhile. Regardless, this should be
reverted, it needs more iteration before it is correct.