rewriting: Make modify_block_argument_type modify_value_type

[PapyChacal]

This requires absolutely no change in itself, I'm not sure why it was typed BlockArgument specifically?

Also, consider modifying a block argument's type as a modification of the parent operation, if any. Is anyone finding it problematic? It didn't trigger any problem on tested stuff, and it's natural for my use-case; stacked PR to come.
No big deal if it's an alarm, I'll hoist it in my user code.

Note/Usecase: Was intended for:

• transformations: stencil shape inference polish #3046

But I meanwhile just including the logic in my own files instead of waiting on core.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.89%. Comparing base (b12d2a5) to head (6d140ef).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3045      +/-   ##
==========================================
- Coverage   89.89%   89.89%   -0.01%     
==========================================
  Files         416      416              
  Lines       52173    52168       -5     
  Branches     8063     8062       -1     
==========================================
- Hits        46903    46898       -5     
- Misses       3977     3978       +1     
+ Partials     1293     1292       -1     

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[superlopuh]

I would say that the code looks good, but would love to hear @math-fehr's opinion on the change before merging