[RISCV] Unaligned vec mem => prefer alt opc vec

Return `true` in `RISCVTTIImpl::preferAlternateOpcodeVectorization` if
subtarget supports unaligned memory accesses.

Author: Mikhail Gudim

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp

@@ -2731,6 +2731,10 @@ unsigned RISCVTTIImpl::getMinTripCountTailFoldingThreshold() const {
   return RVVMinTripCount;
 }
 
+bool RISCVTTIImpl::preferAlternateOpcodeVectorization() const {
+  return ST->enableUnalignedVectorMem();
+}
+
 TTI::AddressingModeKind
 RISCVTTIImpl::getPreferredAddressingMode(const Loop *L,
                                          ScalarEvolution *SE) const {

llvm/lib/Target/RISCV/RISCVTargetTransformInfo.h

@@ -132,7 +132,7 @@ class RISCVTTIImpl final : public BasicTTIImplBase<RISCVTTIImpl> {
 
   unsigned getMaximumVF(unsigned ElemWidth, unsigned Opcode) const override;
 
-  bool preferAlternateOpcodeVectorization() const override { return false; }
+  bool preferAlternateOpcodeVectorization() const override;
 
   bool preferEpilogueVectorization() const override {
     // Epilogue vectorization is usually unprofitable - tail folding or