[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