[NVPTX] Refactor generation of MMA intrinsics and instructions. NFC.

Generalized constructions of 'fragments' of MMA operations to provide
common primitives for construction of the ops. This will make it easier
to add new variants of the instructions that operate on integer types.

Use nested foreach loops which makes it possible to better control
naming of the intrinsics.

This patch does not affect LLVM's output, so there are no test changes.

Differential Revision: https://reviews.llvm.org/D59389

llvm-svn: 359245