[X86] Fix a couple lowering functions that called ReplaceAllUsesOfValueWith...

[X86] Fix a couple lowering functions that called ReplaceAllUsesOfValueWith for the newly created code and then return SDValue(). Use MERGE_VALUES instead.

Returning SDValue() makes the caller think custom lowering was unsuccessful and then it will fall back to trying to expand the original node. This expanded code will end up with no users and end up being pruned later. But it was useless unnecessary work to create it.

Instead return a MERGE_VALUES with all the results so the caller knows something changed. The caller can handle the replacements.

For one of the cases I had to use UNDEF has a dummy value for a result we know is unused. This should get pruned later.

llvm-svn: 357935