[RISCV] Use ZeroOrNegativeOneBooleanContent for vectors.

[craig.topper]

Summary

We use i1 vectors for boolean results for V so the difference between
one and negative probably doesn't matter in most cases. Zero or negative
one is consistent with the P extension if we ever need V and P to co-exist.

[frasercrmck]

I'm not familiar enough with all the decisions this affects but I agree that it should be the same. Have you seen any changes introduced by this? I'm assuming you may have tried it downstream, for example.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

if (Subtarget.hasStdExtA()) {
     setMinCmpXchgSizeInBits(32);
   } else {
     setMaxAtomicSizeInBitsSupported(0);
   }
 
   setBooleanContents(ZeroOrOneBooleanContent);
 
   if (Subtarget.hasStdExtV()) {
-    setBooleanVectorContents(ZeroOrOneBooleanContent);
+    setBooleanVectorContents(ZeroOrNegativeOneBooleanContent);
 
     setOperationAction(ISD::VSCALE, XLenVT, Custom);
 
     // RVV intrinsics may have illegal operands.
     // We also need to custom legalize vmv.x.s.
     setOperationAction(ISD::INTRINSIC_WO_CHAIN, MVT::i8, Custom);
     setOperationAction(ISD::INTRINSIC_WO_CHAIN, MVT::i16, Custom);
     setOperationAction(ISD::INTRINSIC_W_CHAIN, MVT::i8, Custom);