ssa: Create TExprNode even if its read has no reaching def

C++ has a subtle dependency on this behavior.

Previously, when a single TExprNode corresponded to multiple variables,
it would be materialised if just one of those variables had a reaching
definition.

Now the node has been split up, which generally works fine for C++, but
in some cases one of the split-off nodes is missing its reaching def,
but C++ still needs to find the flow to its next use, and therefore
needs the node to be materialised.

Author: asgerf

shared/ssa/codeql/ssa/Ssa.qll

@@ -1681,9 +1681,9 @@ module Make<
     private newtype TNode =
       TWriteDefSource(WriteDefinition def) { DfInput::ssaDefHasSource(def) } or
       TExprNode(DfInput::Expr e, SourceVariable v, Boolean isPost) {
-        exists(Definition def |
-          def.getSourceVariable() = v and
-          e = DfInput::getARead(def)
+        exists(BasicBlock bb, int i |
+          e.hasCfgNode(bb, i) and
+          variableRead(bb, i, v, true)
         )
       } or
       TSsaDefinitionNode(DefinitionExt def) {