[NVPTX] Added support for .f16x2 instructions.

This patch enables support for .f16x2 operations.

Added new register type Float16x2.
Added support for .f16x2 instructions.
Added handling of vectorized loads/stores of v2f16 values.

Differential Revision: https://reviews.llvm.org/D30057
Differential Revision: https://reviews.llvm.org/D30310

llvm-svn: 296032