BitstreamWriter: Fix integer overflow.

BitstreamWriter: Fix integer overflow.

We were previously overflowing a 32-bit multiply operation when emitting large
(>512MB) bitcode files, resulting in corrupted bitcode. Fix by extending
one of the operands to 64 bits.

There are a few other 32-bit integer types in this code that seem like they
also ought to be extended to 64 bits; this will be done separately.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251323 91177308-0d34-0410-b5e6-96231b3b80d8