[llvm] [SimplifyCFG] Don't use a mask for lookup tables generated from switches with an unreachable default case (PR #94468)

[via llvm-commits]

DaMatrix wrote:

> Please rebase your PR, see: [07b9d23](https://github.com/llvm/llvm-project/commit/07b9d231ff9baa6473b0dd588a3ce5330d3e4871).

@DianQK I've rebased onto that, but it doesn't affect this PR since `SimplifyCFG/X86/switch_to_lookup_table.ll` doesn't appear to have been regenerated by that commit.

https://github.com/llvm/llvm-project/pull/94468