Scheduler / Regalloc: use unique_ptr[] instead of std::vector

Scheduler / Regalloc: use unique_ptr[] instead of std::vector

vector.resize() is significantly slower than memset in many STLs
and the cost of initializing these vectors is significant on targets
with many registers. Since we don't need the overhead of a vector,
use a simple unique_ptr instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254526 91177308-0d34-0410-b5e6-96231b3b80d8