[RISCV] Return a vXi1 vector type from getSetCCResultType if V extension is enabled.

nvxXi1 types are legal with V extension and that's the result
vmseq/vmsne/vmslt/etc instructions return.

No test cases yet because the setcc isel patterns aren't in
and we'll need more than basic tests to observe this. I locally
tested that this plus D947078, D94168, D94142, and D94149
was enough to be able to handle the overflow result from
llvm.sadd.overflow.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -398,6 +398,8 @@ EVT RISCVTargetLowering::getSetCCResultType(const DataLayout &DL, LLVMContext &,
                                             EVT VT) const {
   if (!VT.isVector())
     return getPointerTy(DL);
+  if (Subtarget.hasStdExtV())
+    return MVT::getVectorVT(MVT::i1, VT.getVectorElementCount());
   return VT.changeVectorElementTypeToInteger();
 }