Avoid unintended integer overflow in bitstream handling

This change fixes two cases detected by static analysis where an operation is being performed on a 32-bit integer which may overflow and the result is assigned to a 64-bit integer.

Differential Revision: https://reviews.llvm.org/D144661

Author: Andy Kaylor

llvm/include/llvm/Bitstream/BitstreamReader.h

@@ -115,7 +115,7 @@ class SimpleBitstreamCursor {
 
   /// Return the bit # of the bit we are reading.
   uint64_t GetCurrentBitNo() const {
-    return NextChar*CHAR_BIT - BitsInCurWord;
+    return uint64_t(NextChar)*CHAR_BIT - BitsInCurWord;
   }
 
   // Return the byte # of the current bit.

llvm/include/llvm/Bitstream/BitstreamWriter.h

@@ -111,7 +111,7 @@ class BitstreamWriter {
   /// valid. Flushing only occurs at (sub)block boundaries.
   BitstreamWriter(SmallVectorImpl<char> &O, raw_fd_stream *FS = nullptr,
                   uint32_t FlushThreshold = 512)
-      : Out(O), FS(FS), FlushThreshold(FlushThreshold << 20), CurBit(0),
+      : Out(O), FS(FS), FlushThreshold(uint64_t(FlushThreshold) << 20), CurBit(0),
         CurValue(0), CurCodeSize(2) {}
 
   ~BitstreamWriter() {