[RISCV] Mark subvector extracts from index 0 as cheap (#134101)

Previously we only marked fixed length vector extracts as cheap, so this
extends it to any extract at index 0 which should just be a subreg
extract.

This allows extracts of i1 vectors to be considered for DAG combines,
but also scalable vectors too.

This causes some slight improvements with large legalized fixed-length
vectors, but the underlying motiviation for this is to actually prevent
an unprofitable DAG combine on a scalable vector in an upcoming patch.

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -2250,6 +2250,10 @@ bool RISCVTargetLowering::isExtractSubvectorCheap(EVT ResVT, EVT SrcVT,
   if (!isOperationLegalOrCustom(ISD::EXTRACT_SUBVECTOR, ResVT))
     return false;
 
+  // Extracts from index 0 are just subreg extracts.
+  if (Index == 0)
+    return true;
+
   // Only support extracting a fixed from a fixed vector for now.
   if (ResVT.isScalableVector() || SrcVT.isScalableVector())
     return false;