R600/SI: Add pattern for truncating i32 to i1

Fixes half a dozen piglit tests with radeonsi.

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200283 91177308-0d34-0410-b5e6-96231b3b80d8