[RISCV] Don't enable loop vectorizer interleaving if the V extension isn't enabled.

[craig.topper]

Summary

This can cause the vectorizer to generate interleaved scalar
code which might be ok for some CPUs, but definitely not all.
Disable it to restore the previous scalar behavior.

[luke957]

LGTM

llvm/lib/Target/RISCV/RISCVSubtarget.h

public:
   bool enableSaveRestore() const { return EnableSaveRestore; }
   MVT getXLenVT() const { return XLenVT; }
   unsigned getXLen() const { return XLen; }
   RISCVABI::ABI getTargetABI() const { return TargetABI; }
   bool isRegisterReservedByUser(Register i) const {
     assert(i < RISCV::NUM_TARGET_REGS && "Register out of range");
     return UserReservedRegister[i];
   }
-  unsigned getMaxInterleaveFactor() const { return MaxInterleaveFactor; }
+  unsigned getMaxInterleaveFactor() const {
+    return hasStdExtV() ? MaxInterleaveFactor : 1;
+  }
 
 protected:
   // GlobalISel related APIs.
   std::unique_ptr<CallLowering> CallLoweringInfo;
   std::unique_ptr<InstructionSelector> InstSelector;
   std::unique_ptr<LegalizerInfo> Legalizer;
   std::unique_ptr<RegisterBankInfo> RegBankInfo;