[SimplifyCFG] Eliminate dead edges of switches according to the domain of conditions (#165748) In simplifycfg/cvp/sccp, we eliminate dead edges of switches according to the knownbits/range info of conditions. However, these approximations may not meet the real-world needs when the domain of condition values is sparse. For example, if the condition can only be either -3 or 3, we cannot prove that the condition never evaluates to 1 (knownbits: ???????1, range: [-3, 4)). This patch adds a helper function `collectPossibleValues` to enumerate all the possible values of V. To fix the motivating issue, `eliminateDeadSwitchCases` will use the result to remove dead edges. Note: In https://discourse.llvm.org/t/missed-optimization-due-to-overflow-check/88700 I proposed a new value lattice kind to represent such values. But I find it hard to apply because the transition becomes much complicated. Compile-time impact looks neutral: https://llvm-compile-time-tracker.com/compare.php?from=32d6b2139a6c8f79e074e8c6cfe0cc9e79c4c0c8&to=e47c26e3f1bf9eb062684dda4fafce58438e994b&stat=instructions:u This patch removes many dead error-handling codes: https://github.com/dtcxzyw/llvm-opt-benchmark/pull/3012 Closes https://github.com/llvm/llvm-project/issues/165179.
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