[AMDGPU] Minor change to d16 buffer load implementation

Summary:
By not reconstructing the operand list of the SDNode, this change makes
it easier to add the forthcoming new tbuffer and buffer intrinsics.

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

Differential Revision: https://reviews.llvm.org/D49995

Change-Id: I0cb79ef0801532645d7dd954a6d7355139db7b38
llvm-svn: 338784