[AMDGPU] Turn register pressure estimation into forward tracker

[AMDGPU] Turn register pressure estimation into forward tracker

This factors register pressure estimation mechanism from the
GCNSchedStrategy into the forward tracker to unify interface
with other strategies and expose it to other interested phases.

Differential Revision: https://reviews.llvm.org/D33105

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303179 91177308-0d34-0410-b5e6-96231b3b80d8