[RISCV] Correct alignment settings for vector registers.

[HsiangKai]

Summary

According to "9. Vector Memory Alignment Constraints" in V specification, the alignment of vector memory access is aligned to the size of the element. In our current implementation, we support ELEN up to 64. We could assume the alignment of vector registers is 64 under the assumption.

[frasercrmck]

Makes sense to me. I think we'll have to handle "misaligned" vectors at some point too, since the DataLayout can only represent the alignment of vectors based on their overall size.

llvm/lib/Target/RISCV/RISCVRegisterInfo.td

let RegAltNameIndices = [ABIRegAltName] in {
   def VL     : RISCVReg<0, "vl", ["vl"]>;
   def VXSAT  : RISCVReg<0, "vxsat", ["vxsat"]>;
   def VXRM   : RISCVReg<0, "vxrm", ["vxrm"]>;
 }
 
 class VReg<list<ValueType> regTypes, dag regList, int Vlmul>
   : RegisterClass<"RISCV",
                   regTypes,
-                  // FIXME: Spill alignment set to 16 bytes.
-                  128,
+                  64, // The maximum supported ELEN is 64.
                   regList> {
   int VLMul = Vlmul;
-  int Size = !mul(Vlmul, 64); // FIXME: assuming ELEN=64
+  int Size = !mul(Vlmul, 64);
 }
 
 def VR : VReg<[vint8mf2_t, vint8mf4_t, vint8mf8_t,
                vint16mf2_t, vint16mf4_t, vint32mf2_t,
                vint8m1_t, vint16m1_t, vint32m1_t, vint64m1_t,
                vfloat16mf4_t, vfloat16mf2_t, vfloat16m1_t,
                vfloat32mf2_t, vfloat32m1_t, vfloat64m1_t,
                vbool64_t, vbool32_t, vbool16_t, vbool8_t, vbool4_t,