[NVPTX] Fix sign/zero-extending ldg/ldu instruction selection

Summary:
We don't have sign-/zero-extending ldg/ldu instructions defined,
so we need to emulate them with explicit CVTs. We were originally
handling the i8 case, but not any other cases.

Fixes PR26185

Reviewers: jingyue, jlebar

Subscribers: jholewinski

Differential Revision: http://reviews.llvm.org/D19615

llvm-svn: 268272