Scalar lowering for i8x16, i16x8, i32x4 bitmask.

Depending on which lane we are lowering, we can either shift the MSB
into the correct final bit position, then do a big OR of all the nodes.

Bug: v8:10308
Change-Id: Iddf6c077b5a8658a487cef59f2e3bbae3c8bd98d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2219327Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68491}