[NVPTX] Lower loads from global constants using ld.global.nc (aka LDG).

Summary:
After D43914, loads from global variables in addrspace(1) happen with
ld.global.  But since they're constants, even better would be to use
ld.global.nc, aka ldg.

Reviewers: tra

Subscribers: jholewinski, sanjoy, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D43915

llvm-svn: 326390