STRUCT_LIST and INSTANCE_TYPE_LIST are now forced to have the same order
as the InstanceType enum.
Drive-by-fix 1: Move type check and cast functions closer together in objects-inl.h
Drive-by-fix 2: Remove unused instance types SIGNATURE_INFO_TYPE and TYPE_SWITCH_INFO_TYPE.
BUG=
Review-Url: https://codereview.chromium.org/2578573002
Cr-Commit-Position: refs/heads/master@{#41804}
