[InstCombine] Fix -Wimplicit-fallthrough in InstCombinerImpl::visitCallInst (NFC)

/data/llvm-project/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp:3078:3: error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
  default:
  ^
/data/llvm-project/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp:3078:3: note: insert 'break;' to avoid fall-through
  default:
  ^
  break;
1 error generated.