irverify: Enforce invariant that PhiNodes are at the beginning of a BB (#50158)

We have an invariant that all PhiNodes are at the beginning of a BasicBlock
(only possible interrupted by a `nothing`) and we rely on this in various
places for correctness. However, we did not actually verify this invariant.

Author: Keno

base/compiler/ssair/verify.jl

@@ -187,20 +187,30 @@ function verify_ir(ir::IRCode, print::Bool=true,
             end
         end
     end
+    lastbb = 0
+    is_phinode_block = false
     for (bb, idx) in bbidxiter(ir)
+        if bb != lastbb
+            is_phinode_block = true
+            lastbb = bb
+        end
         # We allow invalid IR in dead code to avoid passes having to detect when
         # they're generating dead code.
         bb_unreachable(domtree, bb) && continue
         stmt = ir.stmts[idx][:inst]
         stmt === nothing && continue
         if isa(stmt, PhiNode)
+            if !is_phinode_block
+                @verify_error "φ node $idx is not at the beginning of the basic block $bb"
+                error("")
+            end
             @assert length(stmt.edges) == length(stmt.values)
             for i = 1:length(stmt.edges)
                 edge = stmt.edges[i]
                 for j = (i+1):length(stmt.edges)
                     edge′ = stmt.edges[j]
                     if edge == edge′
-                        # TODO: Move `unique` to Core.Compiler. For now we assume the predecessor list is
+                        # TODO: Move `unique` to Core.Compiler. For now we assume the predecessor list is always unique.
                         @verify_error "Edge list φ node $idx in bb $bb not unique (double edge?)"
                         error("")
                     end
@@ -233,7 +243,14 @@ function verify_ir(ir::IRCode, print::Bool=true,
                 end
                 check_op(ir, domtree, val, Int(edge), last(ir.cfg.blocks[stmt.edges[i]].stmts)+1, idx, print, false, i, allow_frontend_forms)
             end
-        elseif isa(stmt, PhiCNode)
+            continue
+        elseif stmt === nothing
+            # Nothing to do
+            continue
+        end
+
+        is_phinode_block = false
+        if isa(stmt, PhiCNode)
             for i = 1:length(stmt.values)
                 val = stmt.values[i]
                 if !isa(val, SSAValue)