[X86][CodeGen] Do not add `offset` for memory reference using symbol

In the past, D71436 added writing the `offset` operator for some
legitimate cases. However, for memory references in Intel syntax, the
`offset` operator (`[offset sym]`) appears to be superfluous at best,
possibly wrong and contradictory at worst.

This patch bypasses writing the `offset` operator in
`X86AsmPrinter::PrintIntelMemReference` which affects exactly this
case. A similar code flow exists in `X86IntelInstPrinter.cpp` -
`X86IntelInstPrinter::printMemReference`.

The motivation for fixing this output is to allow us to reject the
confusing `call [offset fn_ref]` syntax in MC, as discussed in D149579.

Depends on D149579

Differential Revision: https://reviews.llvm.org/D150047