[TableGen] Use return value from EmitVBRValue instead of calling GetVBRSize on the...

[TableGen] Use return value from EmitVBRValue instead of calling GetVBRSize on the same value. Consistently use unsigned for child sizes. NFCI

getSize and setSize both use unsigned. So size_t doesn't
increase range here and might get truncated if passed to
setSize.

Also not sure why EmitVBRValue was returning uint64_t, but used
an unsigned to supply the value.