[Indvars][NFCI] Remove meaningless restrictive code in IndVars

Variable ExtendOperExpr only exists to check whether it is a SCEV ext.
We create it as SCEV ext right here, so semantically this check is
trivially true. In theory, it may fail if SCEV is smart enough and can
simplify the expression. However, no matter whether it is an ext or not,
we never use this fact for further reasoning. So this code is currently
useless and in theory may become harmful with SCEV's development.

We do not expect any behavior changes with removing it. If it caused
negative changes, the patch should be reverted.