[NVPTX] Use untyped (.b) integer registers in PTX.

This bring LLVM-generated PTX closer to what nvcc generates and avoids
triggering issues in ptxas.

For instance, ptxas does not accept .s16 (or .u16) registers as operands
for .fp16 instructions.

Differential Revision: https://reviews.llvm.org/D23460

llvm-svn: 278568