cmd/w6c/cgen+test: skip dead TRYPROP propret on same-shape ?

Per CLAUDE.md rule 10, align cstage down to wwstage — when every
variant in a `?` propagation maps to itself, the remap loop emits
zero JMPs and the propret label is dead. Lazy-allocate it so the
label-counter ID is only consumed when at least one JMP fires.

Smoke test selfhost/test/trypromote.ww exercises same-shape
(i64|nomem)→(i64|nomem) propagation; cstage and wwstage now emit
byte-identical asm for the TRYPROP region.

.gitignore

@@ -10,6 +10,7 @@
 /loop
 /sym_link
 /memiotest
+/trypromote
 
 # Per-module build artifacts. The .combined.ww files under selfhost/
 # are intentionally tracked — they're frozen bootstrap inputs.