A few fixes are applied in this CL:

1- Instructions which use UIM in V8 only use bits 16 to 19 inclusive.
2- get_simd_register is set to return a reference and not a copy.
3- On vector extract and insert instructions, UIM could be used
to select specific bytes as starting point which may not reflect a lane.
Vector splat uses UIM as a lane selector which remains
unchanged in this CL.

Change-Id: Ieb43afb977dac11d3ea10a2f265c2823f64457e3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3011166Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#75618}