[VPlan] Add vplan native path vectorization test case for inner loop reduction

[VPlan] Add vplan native path vectorization test case for inner loop reduction

Regarding this bug I posted earlier: https://bugs.llvm.org/show_bug.cgi?id=47035

After reading through LLVM source code and getting familiar with VPlan I was able to vectorize the code using by enabling VPlan native path. After talking with @fhahn he suggested that I contribute this as a test case. So here it is. I tried to follow the available guides how to do this best I could. I modified IR code by hand to have more clear variable names instead of numbers.

One thing what I'd like to get input from someone is that is current CHECK lines sufficient enough to verify that the inner loop has been vectorized properly?

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D87564