[AMDGPU] Fix useDeprecatedPositionallyEncodedOperands errors.

This is a follow-on to https://reviews.llvm.org/D134073.

The errors in the R600 half were fixed previously in
https://reviews.llvm.org/D134078. Originally, I thought that the fixes
to the AMDGPU half would be tricky, but upon taking another look,
there were only a couple minor issues that needed fixing:

1. Previously, buffer load instructions (`BUFFER_LOAD_*_LDS_*`) were
populating the `vdata` field in the instruction from the `swz`
operand. This was incorrect, but harmless, as when the LDS option is
set, the instruction does not use the vdata field.

2. The `BUFFER_STORE_LDS_DWORD_gfx90a` instruction was populating
`acc` from the `swz` operand, because `acc` was set to `?`. (I believe
that the intent here was to leave the instruction bit as an "unknown
value", but you can't do that except by setting the bits on `Inst`
directly). Also harmless, for the same reason.

Differential Revision: https://reviews.llvm.org/D140918