[GISel]: Make GlobalISelEmitter rule prioritization compatible with selectionDAG

This patch changes GlobalISelEmitter to rank patterns similar to how the
DAG does it (ie it computes a score for a pattern and adds the added
complexity to it).
This is so that the decision tree for GISelSelector remains compatible
with that of SelectionDAG.

https://reviews.llvm.org/D43270

llvm-svn: 325401