Implement lowering for simd operations I32x4ConvertI16x8 and
I16x8ConvertI8x16. Also, remove skip tests from status files that
were overriden when tests were renamed.

TEST=cctest/test-run-wasm-simd/RunWasm_I16x8ConvertI8x16_turbofan

Change-Id: If428f5039a32995c8ee64294c936419173a87aa7
Reviewed-on: https://chromium-review.googlesource.com/1069007Reviewed-by: Aseem Garg <aseemgarg@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53343}