[flang] Fix -Wstring-conversion in PPCIntrinsicCall.cpp (NFC)

/Users/jiefu/llvm-project/flang/lib/Optimizer/Builder/PPCIntrinsicCall.cpp:617:14: error: implicit conversion turns string literal into bool: 'const char[39]' to 'bool' [-Werror,-Wstring-conversion]
      assert("Invalid vector operation for generator");
      ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~