[RISCV] Remove XLen field from RISCVSubtarget [nfc]

The isRV64 field contains the same information, and we can derive XLen
from that.

Differential Revision: https://reviews.llvm.org/D159306

Author: preames

llvm/lib/Target/RISCV/RISCVSubtarget.cpp

@@ -66,9 +66,6 @@ RISCVSubtarget::initializeSubtargetDependencies(const Triple &TT, StringRef CPU,
     TuneCPU = CPU;
 
   ParseSubtargetFeatures(CPU, TuneCPU, FS);
-  if (Is64Bit)
-    XLen = 64;
-
   TargetABI = RISCVABI::computeTargetABI(TT, getFeatureBits(), ABIName);
   RISCVFeatures::validate(TT, getFeatureBits());
   return *this;

llvm/lib/Target/RISCV/RISCVSubtarget.h

@@ -48,7 +48,6 @@ class RISCVSubtarget : public RISCVGenSubtargetInfo {
   bool ATTRIBUTE = DEFAULT;
 #include "RISCVGenSubtargetInfo.inc"
 
-  unsigned XLen = 32;
   unsigned ZvlLen = 0;
   unsigned RVVVectorBitsMin;
   unsigned RVVVectorBitsMax;
@@ -127,12 +126,10 @@ class RISCVSubtarget : public RISCVGenSubtargetInfo {
   }
   bool is64Bit() const { return IsRV64; }
   MVT getXLenVT() const {
-    return MVT::getIntegerVT(getXLen());
+    return is64Bit() ? MVT::i64 : MVT::i32;
   }
   unsigned getXLen() const {
-    assert((XLen == 32 || XLen == 64) &&
-           "unexpected xlen");
-    return XLen;
+    return is64Bit() ? 64 : 32;
   }
   unsigned getFLen() const {
     if (HasStdExtD)