[4.2] Cleanup dead blocks after noreturn folding

[aschwaighofer]

Follow-up passes might not handle blocks with undef instruction operands
well.

rdar://41139395

[aschwaighofer]

@swift-ci Please test and merge

[atrick]

lgtm

[aschwaighofer]

This failure looks very unrelated to the changes in this PR:

...
3:49:16 29 swift                    0x00000001082dfe52 swift::FileUnit::forAllVisibleModules(llvm::function_ref<bool (std::__1::pair<llvm::ArrayRef<std::__1::pair<swift::Identifier, swift::SourceLoc> >, swift::ModuleDecl*>)>) + 82
13:49:16 30 swift                    0x0000000107fd41c4 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, swift::OptionSet<swift::TypeCheckingFlags, unsigned int>, unsigned int, unsigned int, unsigned int, unsigned int, unsigned int) + 1588
13:49:16 31 swift                    0x0000000107bfa911 swift::CompilerInstance::parseAndCheckTypes(swift::CompilerInstance::ImplicitImports const&) + 961
13:49:16 32 swift                    0x0000000107bfa0f8 swift::CompilerInstance::performSema() + 472
13:49:16 33 swift                    0x00000001070002b2 performCompile(swift::CompilerInstance&, swift::CompilerInvocation&, llvm::ArrayRef<char const*>, int&, swift::FrontendObserver*, swift::UnifiedStatsReporter*) + 1410
13:49:16 34 swift                    0x0000000106ffed12 swift::performFrontend(llvm::ArrayRef<char const*>, char const*, void*, swift::FrontendObserver*) + 2962

[aschwaighofer]

Maybe this needs a clean test?

Unknown kind for DeclBaseName
13:49:16 
UNREACHABLE executed at /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-4.2-branch/swift/lib/ClangImporter/SwiftLookupTable.cpp:1250!
13:49:16 0  swift                    0x000000010ab002f8 llvm::sys::PrintStackTrace(llvm::raw_ostream&) + 40
13:49:16 1  swift                    0x000000010aaff266 llvm::sys::RunSignalHandlers() + 198
13:49:16 2  swift                    0x000000010ab00982 SignalHandler(int) + 258
13:49:16 3  libsystem_platform.dylib 0x00007fff6780bf5a _sigtramp + 26
13:49:16 4  libsystem_platform.dylib 0x00007ffee8c426e8 _sigtramp + 2168678312
13:49:16 5  libsystem_c.dylib        0x00007fff675a91ae abort + 127
13:49:16 6  swift                    0x000000010aa97cce llvm::llvm_unreachable_internal(char const*, char const*, unsigned int) + 542
13:49:16 7  swift                    0x00000001081250c9 swift::SwiftLookupTableReader::lookup(swift::SerializedSwiftName, llvm::SmallVectorImpl<swift::SwiftLookupTable::FullTableEntry>&) 
...

[aschwaighofer]

@swift-ci Please clean test