[MC, NVPTX] Fix warnings

This patch fixes:

  llvm/include/llvm/MC/MCRegisterInfo.h:146:7: error:
  'llvm::MCRegisterInfo' has virtual functions but non-virtual
  destructor [-Werror,-Wnon-virtual-dtor]

  llvm/lib/Target/NVPTX/NVPTXRegisterInfo.cpp:163:21: error:
  comparison of integers of different signs: 'int' and 'size_type'
  (aka 'unsigned long') [-Werror,-Wsign-compare]

Author: kazutakahirata

llvm/include/llvm/MC/MCRegisterInfo.h

@@ -268,6 +268,8 @@ class MCRegisterInfo {
   friend class MCRegUnitRootIterator;
   friend class MCRegAliasIterator;
 
+  virtual ~MCRegisterInfo() {}
+
   /// Initialize MCRegisterInfo, called by TableGen
   /// auto-generated routines. *DO NOT USE*.
   void InitMCRegisterInfo(const MCRegisterDesc *D, unsigned NR, unsigned RA,

llvm/lib/Target/NVPTX/NVPTXRegisterInfo.cpp

@@ -160,11 +160,8 @@ static uint64_t encodeRegisterForDwarf(std::string registerName) {
   // number, which is stored in ULEB128, but in practice must be no more than 8
   // bytes (excluding null terminator, which is not included).
   uint64_t result = 0;
-  for (int i = 0; i < registerName.length(); ++i) {
-    result = result << 8;
-    unsigned char c = registerName[i];
-    result |= c;
-  }
+  for (unsigned char c : registerName)
+    result = (result << 8) | c;
   return result;
 }