[TableGen] Properly calculate the minimum size needed or ConvertFn in GenAsmmatcher.inc files

We were using the number of Matchables rather than the number of rows in the converter table.

This only matters for a few of the targets where the number of matchables is more than 255, but the number of converters is less than 255. Many of the targets have more than 256 converters. So already required a uint16_t.

llvm-svn: 357527