[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