Rewrite assert to avoid warning when the record element type is byte-sized.

BitstreamWriter asserts that when blob data is written from the record
element vector, each element fits in a byte. However, if the record
elements are specified as a SmallVector of 'char', this causes a warning
from -Wtautological-constant-out-of-range-compare. Fix this by using
llvm::isUInt<8> instead of a plain comparison against 256.