Limits #117900 to only fold when scalar_to_vector doesn't perform implicit truncation, as the scaled shift calculation doesn't currently account for this - this can be addressed in a future update.
Fixes#121306
When extracting a smaller integer from a scalar_to_vector source, we were limited to only folding/truncating the lowest bits of the scalar source.
This patch extends the fold to handle extraction of any other element, by right shifting the source before truncation.
Fixes a regression from #117884
The LIT test cases were migrated with the script provided by
Nikita Popov. Due to the size of the change it is split into
several parts.
Reviewed By: nemanja, nikic
Differential Revision: https://reviews.llvm.org/D135474
