Fix for llvm-dis/llvm-bcanalyzer overflows

Summary:
These tools failed for a very large bitcode file produced by LTO due to
64-bit values being assigned to 32-bit types. For the BitstreamReader.h
fix, the value initially fit into the 32-bit unsigned, but there was an
overflow when multiplying by 32 furter below to compute the bit offset.

No test case in the patch as this requires a huge bitcode file.

Reviewers: pcc, george.karpenkov

Subscribers: mehdi_amini, a.sidorin, llvm-commits

Differential Revision: https://reviews.llvm.org/D47731

llvm-svn: 333942