[PBQP] Callee saved regs should have a higher cost than scratch regs

Registers are not all equal. Some are not allocatable (infinite cost),
some have to be preserved but can be used, and some others are just free
to use.

Ensure there is a cost hierarchy reflecting this fact, so that the
allocator will favor scratch registers over callee-saved registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221293 91177308-0d34-0410-b5e6-96231b3b80d8