Remove old/non-working SpillPointers pass (#3261)

And associated stack.h.  The current stack.h clearly doesn't work with
the llvm back as it assumes the stack grows up, which means non of these
has been working or used in a long time.

Rather than trying to fix this unused features its probably cleaner to
just remove it for now and restore it rom git history if its someone
that anyone actually wants to use in the future.

Author: sbc100

CHANGELOG.md

@@ -15,6 +15,10 @@ full changeset diff at the end of each section.
 Current Trunk
 -------------
 
+- Remove old/broken SpollPointers pass.  This pass: Spills values that might be
+  pointers to the C stack. This allows Boehm-style GC to see them properly.
+  This can be revived if needed from git history (#3261).
+
 v98
 ---
 

src/abi/abi.h

@@ -76,7 +76,6 @@ set(passes_SOURCES
   SimplifyGlobals.cpp
   SimplifyLocals.cpp
   Souperify.cpp
-  SpillPointers.cpp
   StackCheck.cpp
   SSAify.cpp
   Untee.cpp