[PBQP] Teach PassConfig to tell if the default register allocator is used.

[PBQP] Teach PassConfig to tell if the default register allocator is used.

This enables targets to adapt their pass pipeline to the register
allocator in use. For example, with the AArch64 backend, using PBQP
with the cortex-a57, the FPLoadBalancing pass is no longer necessary.

llvm-svn: 220321