Change pass registration generated code to allow users to selectively register passes

This slightly tweaks the generated code from:

  #ifdef GEN_PASS_REGISTRATION
  ::mlir::registerPass("flag1", ...
  ::mlir::registerPass("flag2", ...
  #endif // GEN_PASS_REGISTRATION

to:

  #ifdef GEN_PASS_REGISTRATION
  #define GEN_PASS_REGISTRATION_Pass1
  #define GEN_PASS_REGISTRATION_Pass2
  #endif // GEN_PASS_REGISTRATION

  #ifdef GEN_PASS_REGISTRATION_Pass1
  ::mlir::registerPass("flag1", ...
  #endif
  #ifdef GEN_PASS_REGISTRATION_Pass1
  ::mlir::registerPass("flag2", ...
  #endif

That way the generated code can be included by defining the
`GEN_PASS_REGISTRATION` macro as currenty and register all the passes,
but one can also define only `GEN_PASS_REGISTRATION_Pass1` to register a
subset of the passes.

Differential Revision: https://reviews.llvm.org/D77322