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

[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.

llvm-svn: 286342