[AMDGPU] Keep skip branch for ds instructions

[AMDGPU] Keep skip branch for ds instructions

Same as other memory instructions, ds instructions add latency even if
exec is zero. Jumping over them if exec=0 is cheaper than executing
them.
With this change, the branch instruction that skips over a basic block
if exec=0 is not removed when the block contains a ds instruction.

Differential Revision: https://reviews.llvm.org/D97922