[SelectionDAG] Don't convert sextload to zextload through a multi-use freeze (#196700)

Resolves #196590.

The patch https://github.com/llvm/llvm-project/pull/189317 to teach
DAGCombiner to look through freeze incorrectly introduce a miscompile of
sext -> zext. This resolves resolves the miscompile.

GitOrigin-RevId: c2f7e989431dea8eee88be3ba73fb6c89859ca6a