After D147334 we never select _SGPR forms of SMEM instructions on
subtargets that also support the _SGPR_IMM form, so there is no need to
handle them here.
Differential Revision: https://reviews.llvm.org/D149139
Extend SILoadStoreOptimizer to merge into DWORDX8 variant of S_BUFFER_LOAD.
Merging into DWORDX2 and DWORDX4 variants is handled already.
Differential Revision: https://reviews.llvm.org/D108909