IC: next steps (#16729)

* IC: dead code elimination pass
* preparations for a different codegen strategy
* added documentation to the newly written code
* IC: backend code
* IC: backend adjustments
* optimized the compiler a bit
* IC: yet another massive refactoring
* fixes regressions
* cleanups

compiler/cgendata.nim

@@ -112,7 +112,8 @@ type
     isHeaderFile,       # C source file is the header file
     includesStringh,    # C source file already includes ``<string.h>``
     objHasKidsValid     # whether we can rely on tfObjHasKids
-
+    useAliveDataFromDce # use the `alive: IntSet` field instead of
+                        # computing alive data on our own.
 
   BModuleList* = ref object of RootObj
     mainModProcs*, mainModInit*, otherModsInit*, mainDatInit*: Rope
@@ -154,6 +155,7 @@ type
     forwTypeCache*: TypeCache # cache for forward declarations of types
     declaredThings*: IntSet   # things we have declared in this .c file
     declaredProtos*: IntSet   # prototypes we have declared in this .c file
+    alive*: IntSet            # symbol IDs of alive data as computed by `dce.nim`
     headerFiles*: seq[string] # needed headers to include
     typeInfoMarker*: TypeCache # needed for generating type information
     typeInfoMarkerV2*: TypeCache