X86: Split the relocation selection up

Before, we would have conditional operators where one side of the
operator would be of type RelocationTypeAMD64 and the other is of type
RelocationTypeI386.  GCC would noisly warn with -Wenum-compare
diagnostic.

Instead, refactor the code so it is more like the X86 ELF object writer.

llvm-svn: 205752