AMDGPU/SI: Fix kernel argument ABI for HSA

Summary: i8, i16, and f16 values are not extended to 32-bit in the HSA kernel ABI.

Reviewers: arsenm

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, llvm-commits, yaxunl

Differential Revision: https://reviews.llvm.org/D24621

llvm-svn: 281789