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.

llvm-svn: 251323