[BPF] Clean up SelLowering

This patch contains a number of uncontroversial changes:
- Replace all uses of
  `errs`, `assert`, `llvm_unreachable` with `report_fatal_error` with
  informative error strings.
- Replace calls to `fail` in loops with at most one call per error
  instance. Previously a function with 19 arguments would log "too many
  args" 14 times. This was not helpful.
- Change one `if (..) switch ...` to `if (..) { switch ...`. The added
  brace is consistent with a near-identical switch immediately above.
- Elide one `SDValue` copy by using a reference rather than value. This
  is consistent with a variable declared immediately before it.

Reviewed By: yonghong-song

Differential Revision: https://reviews.llvm.org/D156136