AMDGPU: Don't look for DS merge candidates with one use address

The merge is only possible if the base address register is the
same for the two instructions. If there is only the one use,
there's no point in doing an expensive forward scan checking
for memory interference looking for a merge candidate.

This gives a signficant improvement in one extreme testcase.
The code to do the scan is still algorithmically terrible,
so this is still the slowest pass in that example.

llvm-svn: 312096

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