bors 14fbc3c005 Auto merge of #120268 - DianQK:otherwise_is_last_variant_switchs, r=oli-obk
Replace the default branch with an unreachable branch If it is the last variant

Fixes #119520. Fixes #110097.

LLVM currently has limited ability to eliminate dead branches in switches, even with the patch of https://github.com/llvm/llvm-project/issues/73446.

The main reasons are as follows:

- Additional costs are required to calculate the range of values, and there exist many scenarios that cannot be analyzed accurately.
- Matching values by bitwise calculation cannot handle odd branches, nor can it handle values like `-1, 0, 1`. See [SimplifyCFG.cpp#L5424](https://github.com/llvm/llvm-project/blob/llvmorg-17.0.6/llvm/lib/Transforms/Utils/SimplifyCFG.cpp#L5424) and https://llvm.godbolt.org/z/qYMqhvMa8
- The current range information is continuous, even if the metadata for the range is submitted. See [ConstantRange.cpp#L1869-L1870](https://github.com/llvm/llvm-project/blob/llvmorg-17.0.6/llvm/lib/IR/ConstantRange.cpp#L1869-L1870).
- The metadata of the range may be lost in passes such as SROA. See https://rust.godbolt.org/z/e7f87vKMK.

Although we can make improvements, I think it would be more appropriate to put this issue to rustc first. After all, we can easily know the possible values.

Note that we've currently found a slow compilation problem in the presence of unreachable branches. See
https://github.com/llvm/llvm-project/issues/78578.

r? compiler
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The files here use the LLVM FileCheck framework, documented at https://llvm.org/docs/CommandGuide/FileCheck.html.

One extension worth noting is the use of revisions as custom prefixes for FileCheck. If your codegen test has different behavior based on the chosen target or different compiler flags that you want to exercise, you can use a revisions annotation, like so:

// revisions: aaa bbb
// [bbb] compile-flags: --flags-for-bbb

After specifying those variations, you can write different expected, or explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:, like so:

// CHECK: expected code
// aaa-SAME: emitted-only-for-aaa
// aaa-NOT:                        emitted-only-for-bbb
// bbb-NOT:  emitted-only-for-aaa
// bbb-SAME:                       emitted-only-for-bbb