This patch updates VPlan execution to not create and rely on the latch
block of the vector loop existing before VPlan execution.
The patch introduces VPInstruction opcodes ICmpEQ and ExitBranch
to model compares with the canonical IV and the branch exiting the loop
directly in VPlan. This is all subject to change pending feedback.
It is still WIP because there are a few remaining test failures, but I'd
like to share it early so the general direction can be reviewed.