AMDGPU: Avoid constructing new std::vector in initCandidate

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@371136 91177308-0d34-0410-b5e6-96231b3b80d8