ARC Analysis in one pass (#16849)

* Analyse last reads all at once

* Integrate firstWrite analysis

* Small cleanup

* Use sets instead of seqs

* Remove instrTargets

* Reap the benefits

* Implement error diagnostics

* Operate on DFA index for lastRead analysis

* Use mgetOrPut

compiler/ast.nim

@@ -491,6 +491,8 @@ type
     nfDefaultRefsParam # a default param value references another parameter
                        # the flag is applied to proc default values and to calls
     nfExecuteOnReload  # A top-level statement that will be executed during reloads
+    nfLastRead  # this node is a last read
+    nfFirstWrite# this node is a first write
 
   TNodeFlags* = set[TNodeFlag]
   TTypeFlag* = enum   # keep below 32 for efficiency reasons (now: ~40)
@@ -1014,7 +1016,7 @@ const
                                       nfDotSetter, nfDotField,
                                       nfIsRef, nfIsPtr, nfPreventCg, nfLL,
                                       nfFromTemplate, nfDefaultRefsParam,
-                                      nfExecuteOnReload}
+                                      nfExecuteOnReload, nfLastRead, nfFirstWrite}
   namePos* = 0
   patternPos* = 1    # empty except for term rewriting macros
   genericParamsPos* = 2