Auto merge of #57442 - oli-obk:lazy_const, r=RalfJung
Simplify `ConstValue::ScalarPair`
While looking at #57432 I realized that some of our types for representing constants are very big. This reduces `LazyConst` to 3/4th of its original size and simplifies some code around slices at the same time.
r? @RalfJung