AMDGPU: Remove the s_buffer workaround for GFX9 chips

Summary:
I checked the AMD closed source compiler and the workaround is only
needed when x3 is emulated as x4, which we don't do in LLVM.

SMEM x3 opcodes don't exist, and instead there is a possibility to use x4
with the last component being unused. If the last component is out of
buffer bounds and falls on the next 4K page, the hw hangs.

Reviewers: arsenm, nhaehnle

Subscribers: kzhuravl, wdng, yaxunl, dstuttard, tpr, llvm-commits, t-tye

Differential Revision: https://reviews.llvm.org/D42756

llvm-svn: 324486