Not only should SafepointIRVerifier ignore unreachable blocks
(as suggested in https://reviews.llvm.org/D47011) but it also
has to ignore dead blocks.
In @test2 (see the new tests):
br i1 true, label %right, label %left left: ... right: ... merge: %val = phi i8 addrspace(1)* [ ..., %left ], [ ..., %right ] use %val
both left and right branches are reachable.
If they collide then SafepointIRVerifier reports an error.
Because of the foldable branch condition GVN finds the left
branch dead and removes the phi node entry that merges values
from right and left. Then the use comes from the right branch.
This results in no collision.
So, SafepointIRVerifier ends up in different results depending
on either GVN is run or not.
To solve this issue this patch adds Dead Block detection to
SafepointIRVerifier which can ignore dead blocks while
validating IR. The Dead Block detection algorithm is taken
from GVN but modified to not split critical edges. That is
needed to keep CFG unchanged by SafepointIRVerifier.