To allow the X86 verbose assembly to print its informative comments
when used with symbolic disassembly, add a check that the operand
is an immediate and has not been symbolicated to MCExpr operand.

I’m trying to enable the ‘C’ disassembly API option
LLVMDisassembler_Option_SetInstrComments for darwin’s
otool(1) that uses the llvm disassembler API.  The problem is
that the disassembler API can change an immediate operand to
an MCExpr operand if it symbolicates it with the call backs.
And if it does the code in llvm::EmitAnyX86InstComments()
will crash when it assumes these operands are immediates.

The fix for this is very straight forward to just protect the call
to getImm() with a check of isImm().  So if the immediate for
an instruction is symbolicated it simply doesn’t get the X86
verbose assembly comments:

% otool -tV test_asm.o
test_asm.o:
(__TEXT,__text) section
_t1:
0000000000000000	vpshufd	$_t1, %xmm1, %xmm0
0000000000000005	retq
0000000000000006	nopw	%cs:_t1(%rax,%rax)
_t2:
0000000000000010	vpshufd	$-0x1, %xmm0, %xmm0     ## xmm0 = xmm0[3,3,3,3]
0000000000000015	retq
0000000000000016	nopw	%cs:_t1(%rax,%rax)
_t3:
0000000000000020	vpshufd	$_t1, %xmm1, %xmm0
0000000000000025	retq
0000000000000026	nopw	%cs:_t1(%rax,%rax)
_t4:
0000000000000030	vpshufd	$0x2d, %xmm0, %xmm0     ## xmm0 = xmm0[1,3,2,0]
0000000000000035	retq

The fact that the immediate $0x0 is being symbolicated at
all in this case is a different problem which my next patch
will address.

rdar://10989286