[X86][MC] Reject `call`/`jmp [offset fn_ref]` in Intel syntax

[alvinhochun]

Summary

This syntax is confusing and likely invalid. In addition, MASM rejects
it and GAS seems to behave oddly with it. Therefore we shall reject this
syntax for both unconditional call and jmp instructions, as
discussed in D149579.

Depends on D150047

[MaskRay]

See my comment D149579. You can surround the FIXME: MASM does not accept this syntax and GAS assembles this as a direct ... part with .ifdef ERR with llvm-mc ... --defsym ERR=1

[MaskRay]

Looks great!

[MaskRay]

gas feature request: https://sourceware.org/bugzilla/show_bug.cgi?id=30426

[alvinhochun]

Rebase and apply suggestions.

llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp

if (IsUnconditionalBranch) {
           }
         } else {
           if (SM.getSize() == 4 || SM.getSize() == 2)
             MaybeDirectBranchDest = false;
         }
       }
     }
   } else if (IsUnconditionalBranch) {
+    // Treat `call [offset fn_ref]` (or `jmp`) syntax as an error.
+    if (!PtrInOperand && SM.isOffsetOperator())
+      return Error(
+          Start, "`OFFSET` operator cannot be used in an unconditional branch");
     if (PtrInOperand || SM.isBracketUsed())
       MaybeDirectBranchDest = false;
   }
 
   if ((BaseReg || IndexReg || RegNo || DefaultBaseReg != X86::NoRegister))
     Operands.push_back(X86Operand::CreateMem(
         getPointerWidth(), RegNo, Disp, BaseReg, IndexReg, Scale, Start, End,
         Size, DefaultBaseReg, /*SymName=*/StringRef(), /*OpDecl=*/nullptr,

llvm/test/MC/X86/intel-syntax-branch.s

.else
   call dword ptr [offset fn_ref]
   jmp dword ptr [offset fn_ref]
   // CHECK-32-LABEL: t6:
   // CHECK-32: calll *fn_ref
   // CHECK-32: jmpl *fn_ref
 
 .ifdef ERR
 
+  call [offset fn_ref]
+  // ERR-32: {{.*}}.s:[[#@LINE-1]]:8: error: `OFFSET` operator cannot be used in an unconditional branch
+  jmp [offset fn_ref]
+  // ERR-32: {{.*}}.s:[[#@LINE-1]]:7: error: `OFFSET` operator cannot be used in an unconditional branch
+
   call offset fn_ref
   // ERR-32: {{.*}}.s:[[#@LINE-1]]:3: error: invalid operand for instruction
   jmp offset fn_ref
   // ERR-32: {{.*}}.s:[[#@LINE-1]]:3: error: invalid operand for instruction
 
 .endif
 
 .endif