Cleanups to Verifier.

This CL broadens the checks done by the verifier in untyped mode and introduces some subroutines to shorten the code a bit.

Introduce routines CheckUpperIs() CheckUpperMaybe() and CheckValueInputIs() that are called unconditionally by the verifier. If the typing mode is untyped, then don't check anything.

Also added a couple checks for Merge and Loop nodes that catch bugs where the operator and the node disagree on input counts (a bug encountered today).

R=mstarzinger@chromium.org, rossberg@chromium.org
BUG=

Review URL: https://codereview.chromium.org/669073002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24809 ce2b1a6d-e550-0410-aec6-3dcde31c8c00