[NVPTX] Fix bugs related to isSingleValueType

Summary:
With isSingleValueType starting to treat vector types as single-value types,
code that uses this interface needs to be updated.

Test Plan:
vector-global.ll
nvcl-param-align.ll

Reviewers: jholewinski

Reviewed By: jholewinski

Subscribers: llvm-commits, meheff, eliben, jholewinski

Differential Revision: http://reviews.llvm.org/D6573

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224440 91177308-0d34-0410-b5e6-96231b3b80d8