[AMDGPU] Add second pass of the scheduler

[AMDGPU] Add second pass of the scheduler

If during scheduling we have identified that we cannot keep optimistic
occupancy increase critical register pressure limit and try scheduling
of the whole function again. In this case blocks with smaller pressure
will have a chance for better scheduling.

Differential Revision: https://reviews.llvm.org/D30442

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@296506 91177308-0d34-0410-b5e6-96231b3b80d8