LoopVectorize: Don't assume pointees are sized

A pointer's pointee might not be sized: the pointee could be a function.

Report this as IK_NoInduction when calculating isInductionVariable.

This fixes PR21508.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221501 91177308-0d34-0410-b5e6-96231b3b80d8