Only increment the unique \@ ID when it is first used per context (#1030)

This avoids changes to generated `\@` labels just by adding or
removing macros or loops which do not actually use `\@`.

Fixes #1019

test/asm/unique-id-values.asm

@@ -0,0 +1,22 @@
+MACRO m
+  println "  m: \@"
+  for q, \#
+    if q % 2 == 0
+      println "    q = {d:q}: \@"
+    else
+      println "    q = {d:q}"
+    endc
+  endr
+  println "  (m: still \@)"
+ENDM
+
+for p, 10
+  if p % 3 == 0
+    println "p = {d:p}: \@"
+    m 3
+    m 3, 6
+    println " (p: still \@)"
+  else
+    println "p = {d:p}"
+  endc
+endr