[NVPTX] Fix constant expression initializers for global variables

Before this patch the code in printScalarConstant was unable to handle
nested constant expressions like (gep (addrspacecast ptr)) and crashed
with:

LLVM ERROR: Unsupported expression in static initializer:
  addrspacecast ([4 x i8] addrspace(1)* @ga to [4 x i8]*)

We can use lowerConstantForGV instead which is a customized version of
lowerConstant that supports generic() and nested expressions.

Differential Revision: https://reviews.llvm.org/D127878