We've already optimised these, so update the cost model to reflect it.
And skip the isBeforeLegalize check when lowering i8 muls, because it
then misses the cases where, say v32i8, has been type legalised into 2x
v16i8.
Also explicitly disable memory interleaving for any factor other than
two or four.
Constants in BUILD_VECTOR may be down cast into a smaller value that fits LaneBits, i.e., the bit width of elements in the vector.
This cast didn't consider 2^N where it would be cast into -2^N, which still doesn't fit into LaneBits after casting.
This will cause an assertion in later legalization.
2^N should be cast into 0, and this patch reflects such behavior.
This patch also includes a test to reflect the fix.
This patch fixes [issue 61780](https://github.com/llvm/llvm-project/issues/61780)
Related patch: https://reviews.llvm.org/D108669
Reviewed By: tlively
Differential Revision: https://reviews.llvm.org/D147208
