Add a new helper function to IVUsers for returning the "canonical"

Add a new helper function to IVUsers for returning the "canonical"
form of an expression. This is the expression without the
post-increment adjustment made, which is useful in determining
which registers will be used by the expansion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93921 91177308-0d34-0410-b5e6-96231b3b80d8