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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162277 91177308-0d34-0410-b5e6-96231b3b80d8