The !IS_SPEC_OBJECT(x) check implies both IS_STRING(x) and
IS_SIMD_VALUE(x), and generates shorter/better code.  So we
can safely remove the redundant checks.

R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/1278873007

Cr-Commit-Position: refs/heads/master@{#30081}