[AMDGPU] Enable FLAT LDS DMA on gfx9/10 before gfx940

We always had global and scratch loads to LDS in the gfx9,
but did not handle it. These were available via the 'lds'
encoding bit. In gfx940 this bit was reused as 'svs' which
resulted in new '_lds' opcodes effectively pushing this
bit into the opcode, but functionally it is the same. These
instructions are also available on gfx10.

Differential Revision: https://reviews.llvm.org/D125126