PVS-Studio noticed that EmitVBR64 would perform undefined behaviour if the

number of bits was bigger than 32.  I checked every use of this function
that I could find and it looks like the maximum number of bits is 32, so I've
added an assertion checking this property, and a type cast to (hopefully) stop
PVS-Studio from warning about this in the future.

llvm-svn: 162277