[asm] Allow labels as operands in intel asm syntax

This makes a line in llvm/test/CodeGen/X86/asm-block-labels.ll pass
with `asm inteldialect` too.

I don't know if this is something one can hit in practice with inline
asm. The test is from 2007 (4646aa3e) but in 2009 blockaddr was
introduced and e.g. `__asm__ __volatile__("brl %0" :: "X"(&&foo) : "memory");`
compiles to

    call void asm sideeffect "brl $0", "X,..."(i8* blockaddress(@func, %1))

nowadays (thanks to jrtc27 for that example!).

(6c4d255b switched clang to blockaddress on an opt-in basis,
e4801f78 added docs for it, 31b132c0 added IR support.)

I half-heartedly tried to build clang 2.8 locally, but it didn't
just build. And 2.8 didn't have a prebuilt clang binary yet.

The motivation is to make EmitGCCInlineAsmStr() and EmitMSInlineAsmStr()
more alike, and maybe we should delete this code form EmitGCCInlineAsmStr()
instead. But since it's just 3 lines and it's reachable from LLVM IR,
let's do the safer thing for now.

Differential Revision: https://reviews.llvm.org/D114329