create_tmp_table() is a big and convoluted function for creating
internal temporary tables for use by the optimizer.

This patch refactors the function:
- Removes dead code
- Removes usage of goto
- Reduces scope of variables
- Stop reuse of variables for multiple purposes
- Avoids memory allocation until and if needed
- Use more const

The purpose is to make it easier to reason about Field and
Item_field usage for temporary tables.

Change-Id: Idd42649d804816fd7217d87d3cb5363605a24d1e