[AMDGPU] Unroll more to eliminate phis and conditions

Increase threshold to unroll a loop which contains an "if" statement
whose condition defined by a PHI belonging to the loop. This may help
to eliminate if region and potentially even PHI itself, saving on
both divergence and registers used for the PHI.

Add a small bonus for each of such "if" statements.

Differential Revision: https://reviews.llvm.org/D31693

llvm-svn: 299779