Fix op asgn calls with keywords

Examples of such calls:

```ruby
obj[kw: 1] += fo
obj[**kw] &&= bar
```

Before this patch, literal keywords would segfault in the compiler,
and keyword splat usage would result in TypeError.

This handles all cases I can think of:

* literal keywords
* keyword splats
* combined with positional arguments
* combined with regular splats
* both with and without blocks
* both popped and non-popped cases

This also makes sure that to_hash is only called once on the keyword
splat argument, instead of twice, and make sure it is called before
calling to_proc on a passed block.

Fixes [Bug #20051]

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

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