PVS-Studio warning V629 for Clang

uint64_t ReadVBR64(unsigned NumBits) {
  ...
  Result |= uint64_t(Piece & ((1U << (NumBits-1))-1))
            << NextBit;
  ...
}

This suspicious code was found in Clang project by PVS-Studio static code analyzer.
Warning message is:
V629 Consider inspecting the '1U << (NumBits - 1)' expression. Bit shifting of the 32-bit value with a subsequent expansion to the 64-bit type. bitstreamreader.h 362

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