Add an IR instruction to switch over the contents of a union

[kyouko-taiga]

Fixes #1504

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 28 lines in your changes missing coverage. Please review.

Project coverage is 87.65%. Comparing base (211d4e0) to head (29bf165).
Report is 4 commits behind head on main.

Files	Patch %	Lines
Sources/IR/Emitter.swift	61.11%	14 Missing ⚠️
Sources/IR/Operands/Instruction/UnionSwitch.swift	48.14%	14 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1564      +/-   ##
==========================================
- Coverage   87.83%   87.65%   -0.18%     
==========================================
  Files         372      373       +1     
  Lines       22568    22614      +46     
==========================================
+ Hits        19822    19823       +1     
- Misses       2746     2791      +45     

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[kyouko-taiga]

Coverage regression is due to the fact that we don't use the switch instruction anymore and that we only use union_discriminator in synthesized equality operators. We may consider removing both of these instructions but I'd rather do it with separate PRs.