R600/SI: Add pattern for flog2

22 more little piglits with radeonsi.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174615 91177308-0d34-0410-b5e6-96231b3b80d8