[NVPTX] Reduce amount of boilerplate code used to select load instruc…

…tion opcode.

Make opcode selection code for the load instruction a bit easier
to read and maintain.

This patch also catches number of f16 load/store variants that were
not handled before.

Differential Revision: https://reviews.llvm.org/D30513

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296785 91177308-0d34-0410-b5e6-96231b3b80d8