[AVX-512] Use alignedstore256 in patterns that look for stores of the lower 256-bits...

[AVX-512] Use alignedstore256 in patterns that look for stores of the lower 256-bits of a 512-bit vector to use a 256-bit aligned store.

Previously we were only checking for 16 byte alignment instead of 32 byte alignment. Fixes PR30947.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286342 91177308-0d34-0410-b5e6-96231b3b80d8