[X86] Add test cases for failures to form vbroadcastw due to...

[X86] Add test cases for failures to form vbroadcastw due to isTypeDesirableForOp preventing load shrinking to i16.

These are based on existing test cases but use i64 instead of i32.
Some of these end up with i64 zextload/extloads from i16 that we
don't have isel patterns for.

Some of the other cases fail because isTypeDesirableForOp prevents
shrinking the (trunc (i64 (srl (load)))) directly. So we try
to shrink based on the (i64 (srl (load))) but we need 64 - shift_amount
to be a power of 2 to do that shrink.