[SingleSource/Vectorizer] Add unit tests for conditional scalar assignment pattern

[michaelmaitland]

Dedicated unit tests for loops which contain a conditional assignment inside the loop body.

An earlier version of this patch was posted as https://reviews.llvm.org/D144909. Some updates have been made to cover more test cases.

I tested this patch with the CSA vectorization patch applied (I will add a link once the CSA patch is posted) using the following options:

-O3 -mllvm -enable-csa-vectorization -mcpu=sifive-x280

I also ran it with -mllvm -force-tail-folding-style=data-with-evl appended to the previous option set as well as -mllvm -force-tail-folding-style=none.

I also verified that vector code was generated for functions that we are currently able to vectorize using objdump. This patch contains some examples that we are not able to vectorize today but should be able to in the future.

An RFC that describes the class of problems being tested can be found here.

https://github.com/llvm/llvm-project/pull/106560 is the patch that implements CSA vectorization.