[turbofan] Simplify handling of spreads in array literals.
[turbofan] Simplify handling of spreads in array literals.

R=arv@chromium.org
TEST=mjsunit/harmony/spread-array

Review URL: https://codereview.chromium.org/1154873002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#28598}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh