PTX 6.3 extends `wmma` instruction to support s8/u8/s4/u4/b1 -> s32.

All of the new instructions are still handled mostly by tablegen. I've slightly
refactored the code to drive intrinsic/instruction generation from a master
list of supported variants, so all irregularities have to be implemented in one place only.

The test generation script wmma.py has been refactored in a similar way.

Differential Revision: https://reviews.llvm.org/D60015

llvm-svn: 359247