[x86] Teach the asm comment printing to only print the clarification of
an immediate operand when we don't have instruction-specific comments.

This ensures that instruction-specific comments are attached to the same
line as the instruction which is important for using them to write
readable and maintainable tests. My next commit will just such a test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217099 91177308-0d34-0410-b5e6-96231b3b80d8