SwiftTailCC has a different set of requirements than the C calling convention for a tail call. The exact argument sequence doesn't have to match, but fewer ABI-affecting attributes are allowed. Also make sure the musttail diagnostic triggers if a musttail call isn't actually a tail call.
This directory contains testcases that the verifier is supposed to detect as malformed LLVM code. Testcases for situations that the verifier incorrectly identifies as malformed should go in the test/Assembler directory.