Avoid partiality in `Language.Haskell.Syntax.Expr`, `GHC.Hs.Expr`, `GHC.Rename.Expr`, etc.

In particular, use `NonEmpty` where appropriate:
- the argument of `FieldLabelString`
- the argument of `HsMultiIf`
- `grhssGRHSs`

Decreases overall compile-time allocation by about 0.1% in the benchmark suite (min -0.8%, max +0.3%).

Metric Decrease:
    T3294