R600/SI: Fix ftrunc f64 conformance failures.

Re-add the tests since they were deleted at some point

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219036 91177308-0d34-0410-b5e6-96231b3b80d8
R600: Custom lower [s|u]int_to_fp for i64 -> f64

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219037 91177308-0d34-0410-b5e6-96231b3b80d8
R600/SI: Custom lower f64 -> i64 conversions

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219038 91177308-0d34-0410-b5e6-96231b3b80d8