[mypyc] Remove symbol table and value index table from Environment - #9768

Instead of maintaining symbol tables in Environment, they are maintained in
IRBuilder and discarded after they are no longer needed.

I also removed the value index table (indexes) from Environment. It was
mostly used for three purposes:

1. Finding all values in a function
2. Finding argument registers
3. Sorting values in a predictable order

These use cases are now handled differently:

1. I added functions, including all_values, that find values in IR by
   iterating over it and collecting them.
2. FuncIR now stores argument registers explicitly.
3. I added a helper function to produce an ordering for values
   (make_value_ordering).

There are three main reasons why I think that this PR makes sense. First, it
simplifies the IR and makes things easier to understand. Second, transforming
IR is easier and less error-prone, as the symbol tables and index dictionaries
no longer need to be kept in sync with the IR. Third, this makes it easy to get
rid of Environment altogether (in a follow-up PR).

A nice side effect is that we can now just create a Register instance directly
and start using it, instead of having to call alloc_temp().

Some tests had to be updated, since the registers are now numbered differently
in some cases. Arguably, it's now more logical and predictable. There won't be
gaps in the numbering.

Work on mypyc/mypyc#781.