issues/0066: RESOLVED — match-value arms lowered against result type

A value-position match's arms are now lowered with `target_type` set to
the merge's `result_type`, so positive and negated integer literals pick
the same width. Fixes the `PHI node operands are not the same type as the
result` failure for `if n == { case 0: 100; else: -1; }`-style returns.

Regression: examples/0043-basic-match-value-mixed-width.sx.
Gates: zig build, zig build test, run_examples.sh -> 345 passed.

examples/expected/0043-basic-match-value-mixed-width.stdout

@@ -0,0 +1,2 @@
+sign: -1 0 1
+classify: 100 10 -1