Fixes the case when the 'imm' is in range [8000,ffff], as the specification
says the 'imm' is sign-extended before the comparsion, so the effective range
is [max_unsigned-7fff,max_unsigned].

TEST=mjsunit/generated-transition-stub@3b8ad45e
BUG=

Review-Url: https://codereview.chromium.org/2386673002
Cr-Commit-Position: refs/heads/master@{#39922}