JIT: disable phi based redundant branch opts (#76695)

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 #76636, #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