R600: Remove the fpconst64.ll test which was failing on non-x86 buildbots

I'm guessing the failure had something to do with the double precision
floating point constant used in the test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186191 91177308-0d34-0410-b5e6-96231b3b80d8