fixes a critical GC safety inference bug (#10615)

* fixes a critical GC safety inference bug
* make nimsuggest compile again
* make Nimble compile again

tests/effects/tgcsafe3.nim

@@ -0,0 +1,21 @@
+discard """
+  errormsg: "'myproc' is not GC-safe as it accesses 'global_proc' which is a global using GC'ed memory"
+  line: 12
+  cmd: "nim $target --hints:on --threads:on $options $file"
+"""
+
+var useGcMem = "string here"
+
+var global_proc: proc(a: string) {.nimcall.} = proc (a: string) =
+  echo useGcMem
+
+proc myproc(i: int) {.gcsafe.} =
+  when false:
+    if global_proc != nil:
+      echo "a"
+    if isNil(global_proc):
+      return
+
+  global_proc("ho")
+
+myproc(0)