AMDGPU: Avoid constructing new std::vector in initCandidate

Approximately 30% of the time was spent in the std::vector
constructor. In one testcase this pushes the scheduler to being the
second slowest pass.

I'm not sure I understand why these vector are necessary. The default
scheduler initCandidate seems to use some pre-existing vectors for the
pressure.

llvm-svn: 371136