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.
(Phabricator won't let me delete this.)