Relax the clearance calculating for breaking partial register dependency.

Summary: LLVM assumes that large clearance will hide the partial register spill penalty. But in our experiment, 16 clearance is too small. As the inserted XOR is normally fairly cheap, we should have a higher clearance threshold to aggressively insert XORs that is necessary to break partial register dependency.

Reviewers: wmi, davidxl, stoklund, zansari, myatsina, RKSimon, DavidKreitzer, mkuper, joerg, spatel

Subscribers: davidxl, llvm-commits

Differential Revision: http://reviews.llvm.org/D21560

llvm-svn: 274068