[NVPTX] use incomplete aggregate initializers (#79062)

The PTX ISA specifies that initializers may be incomplete ([5.4.4.
Initializers](https://docs.nvidia.com/cuda/parallel-thread-execution/index.html#initializers))
> As in C, array initializers may be incomplete, i.e., the number of
initializer elements may be less than the extent of the corresponding
array dimension, with remaining array locations initialized to the
default value for the specified array type.

Emitting initializers in this form is preferable because it reduces the
size of the PTX, in some cases significantly, and can improve compile
time of ptxas as a result.