[ASDAlexander77]

Fixes #462.

let g: Node | null | undefined = mkU(), where mkU returns Node | undefined, failed in every model with "'ts.Global' op expects region #0 to have 0 or 1 blocks". Every conditional global initializer, such as let x = on() ? 1 : 2, failed the same way.

Widening the optional into the union is a ts.If, just like a conditional. The affine lowering turns that into blocks, and the SingleBlockImplicitTerminator trait on ts.Global rejected them.

Changes

• Drop the trait. MLIRGen still builds one block ending in ts.GlobalResult, and nothing reads the initializer as a single block after the affine lowering.
• Make branching initializers constructors. GlobalOpLowering makes a global whose initializer has more than one block a constructor, because only a function can hold the branches.
• rc: once the program compiled, the widened optional lost its object under rc:
  • The call runs before the ts.If, and the branch yields its value through views (ts.Value, a cast, ts.OptionalValue).
  • So the global never took the call's reference, which was given back at the end of the initializer.
  • claimGlobalInitializers (rc: a global takes the reference of an owned producer through a widening view (#459) #468) now follows a ts.If whose branches all yield either one owned producer from before it, through views, or a value that holds nothing (the empty optional, null, undefined).
  • The global then takes that producer's count.

Tests

• New test: 00global_initializer_branches.ts, registered as compile and JIT tests and in the rc/none corpus. It failed to compile before the change. It covers:
  • an optional widened into a union, set and empty;
  • conditional number, string and object globals.
• Own corpus: unchanged.
• Full Release ctest: 3666/3667. The one failure is test-compile-gc-defaultlib-collector, the known local LNK2005 against the stale installed default library, which fails on main the same way.

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