core: Move SSAValue definitions around (NFC)

[math-fehr]

Stacked PRs:

• core: Make IRDLOperation results typed #3991
• dialects: Raise a parsing error in accfg.setup when result type is invalid #3990
• core: Make OpResult and BlockArgument generic like SSAValue #3989
• core: Add support for typed SSA values in isa #3988
• core: Make SSAValue generic #3987
• ->core: Move SSAValue definitions around (NFC) #3986

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core: Move SSAValue definitions around (NFC)

Move the definition of SSAValue after the definition of Attributes.

[alexarice]

Why though? Edit: nm I get it

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.16%. Comparing base (cca5ed5) to head (85e62b9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3986   +/-   ##
=======================================
  Coverage   90.16%   90.16%           
=======================================
  Files         458      458           
  Lines       58291    58291           
  Branches     5691     5691           
=======================================
+ Hits        52558    52559    +1     
  Misses       4340     4340           
+ Partials     1393     1392    -1     

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