AMDGPU: Use set for tracked registers

The majority of the time spent in the pass checking
for the register reads. Rather than searching all of
the defined registers for uses in each instruction,
use a set of defined registers and check the operands
of the instruction.

This process still is algorithmically not great,
but with the additional trick of skipping the analysis
for addresses with one use, this brings one slow
testcase into a reasonable range.

llvm-svn: 312206