New features and bug fix in MLIR test generation tool

- Option `--variable_names <names>` allows the user to pass names for FileCheck
  regexps representing variables. Variable names are separated by commas, and
  empty names can be used to generate specific variable names automatically.
  For example, `--variable-names arg_0,arg_1,,,result` will produce regexp names
  `ARG_0`, `ARG_1`, `VAR_0`, `VAR_1`, `RESULT`, `VAR_2`, `VAR_3`, ...

- Option '--attribute_names <names>' can be used to generate global regexp names
  to represent attributes. Useful for affine maps. Same behavior as
  '--variable_names'.

- Bug fixed for scope detection of SSA variables in ops with nested regions that
  return SSA values (e.g., 'linalg.generic'). Originally, returned SSA values were
  inserted in the nested scope.

This version of the tool has been used to generate unit tests for the following
patch: https://reviews.llvm.org/D153291

For example, the main body of the test named 'test_select_2d_one_dynamic' was
generated using the following command:

```
$ mlir-opt -pass-pipeline='builtin.module(func.func(tosa-to-linalg))' test_select_2d_one_dynamic.tosa.mlir | generate-test-checks.py --attribute_names map0,map1,map2 --variable_names arg0,arg1,arg2,const1,arg0_dim1,arg1_dim1,,arg2_dim1,max_dim1,,,arg0_broadcast,,,,,,,arg1_broadcast,,,,,,,arg2_broadcast,,,,,,result
```

Reviewed By: eric-k256

Differential Revision: https://reviews.llvm.org/D154458