[llvm] [SelectionDAG] Fix bug related to demanded bits/elts for BITCAST (PR #139085)

[Matt Arsenault via llvm-commits]

================
@@ -4192,6 +4192,15 @@ class TargetLowering : public TargetLoweringBase {
   ///    results of this function, because simply replacing TLO.Old
   ///    with TLO.New will be incorrect when this parameter is true and TLO.Old
   ///    has multiple uses.
+  /// Vector elements that aren't demanded can be turned into poison unless the
+  /// corresponding bi in the \p DoNotPoisonEltMask is set.
----------------
arsenm wrote:

```suggestion
  /// corresponding bit in the \p DoNotPoisonEltMask is set.
```

https://github.com/llvm/llvm-project/pull/139085