[X86][MC] Fix assembling rip-relative addressing + immediate displacements

In the rare case where the input contains rip-relative addressing with
immediate displacements, *and* the instruction ends with an immediate,
we encode the instruction in the wrong way:

movl $12345678, 0x400(%rdi) // all good, no rip-relative addr
movl %eax, 0x400(%rip) // all good, no immediate at the end of the instruction
movl $12345678, 0x400(%rip) // fails, encodes address as 0x3fc(%rip)

Offset is a label:

movl $12345678, foo(%rip)

we want to account for the size of the immediate (in this case,
$12345678, 4 bytes).

Offset is an immediate:

movl $12345678, 0x400(%rip)

we should not account for the size of the immediate, assuming the
immediate offset is what the user wanted.

Differential Revision: https://reviews.llvm.org/D43050

llvm-svn: 324772