cmd/compile: support unsigned induction variables

[unitdevgcc]

This change adds unsigned induction-variable matching to the SSA prove pass.

The compiler now recognizes unsigned Less/Leq loop conditions, tracks the induction variable in the unsigned domain, and checks unsigned wraparound using the actual SSA type width. Unsigned induction variables no longer reach the signed-only downward loop inversion path, so they are conservatively skipped there instead of panicking.

Tests cover unsigned BCE loops with steps 1 and 2, plus ascending and descending unsigned loops in the prove diagnostics.

Fixes #65918

Tests:

./bin/go test ./cmd/compile/internal/...
./run.bash ../test/prove.go ../test/prove_invert_loop_with_unused_iterators.go

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Patch Set 1:

(1 comment)

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