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

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