Create new accessors to get arguments for call/invoke instructions. It breaks

Create new accessors to get arguments for call/invoke instructions. It breaks
encapsulation to force the users of these classes to know about the internal
data structure of the Operands structure. It also can lead to errors, like in
the MSIL writer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@105539 91177308-0d34-0410-b5e6-96231b3b80d8