[RISCV] Use RISCV::RVVBitsPerBlock for RGK_ScalableVector in getRegisterBitWidth.

I might be wrong, but I think this is should be width of the known
min size we use for scalable vectors. It shouldn't scale with
minimum vlen.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D107945

Author: topperc

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h

@@ -66,7 +66,7 @@ class RISCVTTIImpl : public BasicTTIImplBase<RISCVTTIImpl> {
           ST->hasStdExtV() ? ST->getMinRVVVectorSizeInBits() : 0);
     case TargetTransformInfo::RGK_ScalableVector:
       return TypeSize::getScalable(
-          ST->hasStdExtV() ? ST->getMinRVVVectorSizeInBits() : 0);
+          ST->hasStdExtV() ? RISCV::RVVBitsPerBlock : 0);
     }
 
     llvm_unreachable("Unsupported register kind");