Fix a x86-64 codegen deficiency. Allow gv + offset when using rip addressing mode.

Fix a x86-64 codegen deficiency. Allow gv + offset when using rip addressing mode.
Before:
_main:
        subq    $8, %rsp
        leaq    _X(%rip), %rax
        movsd   8(%rax), %xmm1
        movss   _X(%rip), %xmm0
        call    _t
        xorl    %ecx, %ecx
        movl    %ecx, %eax
        addq    $8, %rsp
        ret
Now:
_main:
        subq    $8, %rsp
        movsd   _X+8(%rip), %xmm1
        movss   _X(%rip), %xmm0
        call    _t
        xorl    %ecx, %ecx
        movl    %ecx, %eax
        addq    $8, %rsp
        ret

Notice there is another idiotic codegen issue that needs to be fixed asap:
xorl    %ecx, %ecx
movl    %ecx, %eax

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46850 91177308-0d34-0410-b5e6-96231b3b80d8