Move r186044 tests into CodeGen/X86

I had thought that these tests could be target-neutral, but in practice this is
not the case (on some targets, like Hexagon and Darwin), they trigger an assert
(a different assert than the one that r186044 fixes).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186051 91177308-0d34-0410-b5e6-96231b3b80d8