[MC] Check the upper bound in truncate assertion

[phosek]

Summary

The truncateToSize function already has assertion to check the
lower boundary for the number bytes, but it does not check the
upper boundary which could still lead to usage errors.

[• rafael]

LGTM

llvm/trunk/lib/MC/MCAsmStreamer.cpp

do {
 
     Comments = Comments.substr(Position+1);
   } while (!Comments.empty());
 
   CommentToEmit.clear();
 }
 
 static inline int64_t truncateToSize(int64_t Value, unsigned Bytes) {
-  assert(Bytes && "Invalid size!");
+  assert(Bytes > 0 && Bytes <= 8 && "Invalid size!");
   return Value & ((uint64_t) (int64_t) -1 >> (64 - Bytes * 8));
 }
 
 void MCAsmStreamer::emitRawComment(const Twine &T, bool TabPrefix) {
   if (TabPrefix)
     OS << '\t';
   OS << MAI->getCommentString() << T;
   EmitEOL();