[DAGCombiner] Fix scalarizeExtractedBinOp for some SETCC cases (#123071)
PR https://github.com/llvm/llvm-project/pull/118823 added a
DAG combine for extracting elements of a vector returned from
SETCC, however it doesn't correctly deal with the case where
the vector element type is not i1. In this case we have to
take account of the boolean contents, which are represented
differently between vectors and scalars. The code now
explicitly performs an inreg sign extend in order to get the
same result.
Fixes https://github.com/llvm/llvm-project/issues/121372