JIT: disable phi based redundant branch opts

This is exposing our lack of SSA update and leading downstream opts like CSE
and assertion prop to make bad decisions.

Disabling for now until I have time to figure out how to safely enable.

Fixes dotnet#76636, dotnet#76507

src/coreclr/jit/redundantbranchopts.cpp

@@ -657,7 +657,12 @@ bool Compiler::optRedundantBranch(BasicBlock* const block)
         // We were unable to determine the relop value via dominance checks.
         // See if we can jump thread via phi disambiguation.
         //
-        return optJumpThreadPhi(block, tree, treeNormVN);
+        // optJumpThreadPhi disabled as it is exposing problems with stale SSA.
+        // See issue #76636 and related.
+        //
+        // return optJumpThreadPhi(block, tree, treeNormVN);
+
+        return false;
     }
 
     // Be conservative if there is an exception effect and we're in an EH region