Adds support for bitcasting a 'small element' vector to 'large element' scalar/vector (e.g. v16i8 to v4i32 or v2i32 to i64), which is required for PR30845. We extract the knownbits for each 'small element' part and concatenate the results together.
I have the code ready for 'large element' scalar/vector to 'small element' vector bitcasting as well if you think it worth adding to this patch now before we have any useful test cases.