[TableGen] Allow asm writer to use up to 3 OpInfo tables instead of 2. This...

[TableGen] Allow asm writer to use up to 3 OpInfo tables instead of 2. This allows x86 to use 56 total bits made up of a 32-bit, 16-bit, and 8-bit table. Previously we were using 64 total bits.

This saves 14K from the x86 table size. And saves space on other targets as well.

llvm-svn: 257315