Cleanup and test X86AsmPrinter::printPCRelImm.

It is only used for asm printing.

On X86 we put basic block addresses on register before passing them to inline
asm, so the MO_MachineBasicBlock case was dead.

MO_ExternalSymbol was dead since any symbol being passed to inline asm
is represented as MO_GlobalAddress.

The MO_GlobalAddress and MO_Register cases were not tested.