post-ra-sched: Replace a std::set of regs with a bitvector.

post-ra-sched: Replace a std::set of regs with a bitvector.

Assuming that a single std::set node adds 3 control words, a bitvector
can store (3*8+4)*8=224 registers in the allocated memory of a single
element in the std::set (x86_64). Also we don't have to call malloc
for every register added.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151269 91177308-0d34-0410-b5e6-96231b3b80d8