[MLIR][SPIRVToLLVM] Enhanced conversion for execution mode

This patch introduces a new conversion pattern for `spv.ExecutionMode`.
`spv.ExecutionMode` may contain important information about the entry
point, which we want to preserve. For example, `LocalSize` provides
information about the work-group size that can be reused. Hence, the
pattern for entry-point ops changes to the following:
- `spv.EntryPoint` is still simply removed
- Info from `spv.ExecutionMode` is used to create a global struct variable,
  which looks like:

  ```
  struct {
    int32_t executionMode;
    int32_t values[];          // optional values
  };
  ```

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D89989