[TableGen] Fix warning when compiling generated MCCodeEmitter

[foad]

Summary

This fixes an instance of:
warning: cast from 'const unsigned long *' to 'unsigned char *' drops const qualifier [-Wcast-qual]
when compiling the generated MCCodeEmitter for an out-of-tree target
that uses the optional support for instruction widths > 64 bits.

[dsanders]

LGTM

llvm/utils/TableGen/CodeEmitterGen.cpp

void CodeEmitterGen::run(raw_ostream &o) {
   if (UseAPInt) {
     int NumWords = APInt::getNumWords(BitWidth);
     int NumBytes = (BitWidth + 7) / 8;
     o << "  const unsigned opcode = MI.getOpcode();\n"
       << "  if (Inst.getBitWidth() != " << BitWidth << ")\n"
       << "    Inst = Inst.zext(" << BitWidth << ");\n"
       << "  if (Scratch.getBitWidth() != " << BitWidth << ")\n"
       << "    Scratch = Scratch.zext(" << BitWidth << ");\n"
-      << "  LoadIntFromMemory(Inst, (uint8_t *)&InstBits[opcode * " << NumWords
-      << "], " << NumBytes << ");\n"
+      << "  LoadIntFromMemory(Inst, (const uint8_t *)&InstBits[opcode * "
+      << NumWords << "], " << NumBytes << ");\n"
       << "  APInt &Value = Inst;\n"
       << "  APInt &op = Scratch;\n"
       << "  switch (opcode) {\n";
   } else {
     o << "  const unsigned opcode = MI.getOpcode();\n"
       << "  uint64_t Value = InstBits[opcode];\n"
       << "  uint64_t op = 0;\n"
       << "  (void)op;  // suppress warning\n"