AMDGPU: Stop wasting argument registers with v3i32/v3f32

SelectionDAGBuilder widens v3i32/v3f32 arguments to
to v4i32/v4f32 which consume an additional register.
In addition to wasting argument space, this produces extra
instructions since now it appears the 4th vector component has
a meaningful value to most combines.

llvm-svn: 338197