(Review input addressed)
(Added handling of UPDATE/DELETE and partitioning w/o index)

If the properties of the used collation allow, do the following
equivalent rewrites:

1. UPPER(key_col)=expr  ->  key_col=expr
   expr=UPPER(key_col)  ->  expr=key_col
   (also rewrite both sides of the equality at the same time)

2. UPPER(key_col) IN (constant-list)  -> key_col IN (constant-list)

- Mark utf8mb{3,4}_general_ci as collations that allow this.
- Add optimizer_switch='sargable_casefold=ON' to control this.
  (ON by default in this patch)
- Cover the rewrite in Optimizer Trace, rewrite name is
  "sargable_casefold_removal".