[AMPGPU] Emit s_singleuse_vdst instructions when a register is used multiple...

[AMPGPU] Emit s_singleuse_vdst instructions when a register is used multiple times in the same instruction. (#89601)

Previously, multiple uses of a register within the same instruction were
being counted as multiple uses. This has been corrected to
only count as a single use as per the specification allowing for
more optimisation candidates.