[AMDGPU] Prepare for introduction of v3 and v5 MVTs

AMDGPU would like to have MVTs for v3i32, v3f32, v5i32, v5f32. This
commit does not add them, but makes preparatory changes:

* Fixed assumptions of power-of-2 vector type in kernel arg handling,
  and added v5 kernel arg tests and v3/v5 shader arg tests.

* Added v5 tests for cost analysis.

* Added vec3/vec5 arg test cases.

Some of this patch is from Matt Arsenault, also of AMD.

Differential Revision: https://reviews.llvm.org/D58928

Change-Id: I7279d6b4841464d2080eb255ef3c589e268eabcd
llvm-svn: 356342