C++/C#: Fix phantom Chi definitions in PrintSSA

When `PrintSSA.qll` is imported, IR dumps will be annotated with the alias analysis information used during SSA construction. When printing this information, we incorrectly treated instructions at offset -1, which should only be `Phi` instructions, as `Chi` instructions for the instruction at offset 0. This produced phantom annotations, but did not affect the correctness of the actual IR.

Author: Dave Bartolomeo

cpp/ql/src/semmle/code/cpp/ir/implementation/aliased_ssa/internal/PrintSSA.qll

@@ -6,11 +6,12 @@ private import DebugSSA
 
 bindingset[offset]
 private string getKeySuffixForOffset(int offset) {
+  offset >= 0 and
   if offset % 2 = 0 then result = "" else result = "_Chi"
 }
 
 bindingset[offset]
-private int getIndexForOffset(int offset) { result = offset / 2 }
+private int getIndexForOffset(int offset) { offset >= 0 and result = offset / 2 }
 
 /**
  * Property provide that dumps the memory access of each result. Useful for debugging SSA

cpp/ql/src/semmle/code/cpp/ir/implementation/unaliased_ssa/internal/PrintSSA.qll

@@ -6,11 +6,12 @@ private import DebugSSA
 
 bindingset[offset]
 private string getKeySuffixForOffset(int offset) {
+  offset >= 0 and
   if offset % 2 = 0 then result = "" else result = "_Chi"
 }
 
 bindingset[offset]
-private int getIndexForOffset(int offset) { result = offset / 2 }
+private int getIndexForOffset(int offset) { offset >= 0 and result = offset / 2 }
 
 /**
  * Property provide that dumps the memory access of each result. Useful for debugging SSA

csharp/ql/src/semmle/code/csharp/ir/implementation/unaliased_ssa/internal/PrintSSA.qll

@@ -6,11 +6,12 @@ private import DebugSSA
 
 bindingset[offset]
 private string getKeySuffixForOffset(int offset) {
+  offset >= 0 and
   if offset % 2 = 0 then result = "" else result = "_Chi"
 }
 
 bindingset[offset]
-private int getIndexForOffset(int offset) { result = offset / 2 }
+private int getIndexForOffset(int offset) { offset >= 0 and result = offset / 2 }
 
 /**
  * Property provide that dumps the memory access of each result. Useful for debugging SSA