Add some convenience accessors for the underlying Use of an operand.
These complement many of the existing accessors and make it
significantly easier to write code which needs to poke at the underlying
Use without hard coding the operand number at which it resides for
a particular instruction. No functionality changed of course.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202102 91177308-0d34-0410-b5e6-96231b3b80d8