Avoid creating invalid PhiNodes in IR passes

As of #50158, irverify catches cases where PhiNodes show up in the
middle of a basic block (which is illegal). Unfortunately, it turns
out there were two cases in Base, where we created just such code:

1. When cfg_simplify! merged basic blocks, it didn't bother to delete
   (resp, replace by the one incoming edge) the PhiNodes in the basic
   block it was merging.

2. In irinterp we try to delete instructions that result in constants.
   This is not legal if the instruction is a PhiNode.

The second of these is somewhat unfortunate, but any subsequent compaction
will of course take care of it, so I don't think it's a huge issue
to just disable the replacement.

base/compiler/ssair/irinterp.jl

@@ -154,7 +154,7 @@ function reprocess_instruction!(interp::AbstractInterpreter, idx::Int, bb::Union
     if rt !== nothing
         if isa(rt, Const)
             ir.stmts[idx][:type] = rt
-            if is_inlineable_constant(rt.val) && (ir.stmts[idx][:flag] & IR_FLAG_EFFECT_FREE) != 0
+            if is_inlineable_constant(rt.val) && !isa(inst, PhiNode) && (ir.stmts[idx][:flag] & IR_FLAG_EFFECT_FREE) != 0
                 ir.stmts[idx][:inst] = quoted(rt.val)
             end
             return true

base/compiler/ssair/passes.jl

@@ -2246,6 +2246,7 @@ function cfg_simplify!(ir::IRCode)
     result_idx = 1
     for (idx, orig_bb) in enumerate(result_bbs)
         ms = orig_bb
+        bb_start = true
         while ms != 0
             for i in bbs[ms].stmts
                 node = ir.stmts[i]
@@ -2304,7 +2305,14 @@ function cfg_simplify!(ir::IRCode)
                             isassigned(renamed_values, old_index) && kill_current_use!(compact, renamed_values[old_index])
                         end
                     end
-                    compact.result[compact.result_idx][:inst] = PhiNode(edges, values)
+                    if length(edges) == 0 || (length(edges) == 1 && !isassigned(values, 1))
+                        compact.result[compact.result_idx][:inst] = nothing
+                    elseif length(edges) == 1 && !bb_start
+                        compact.result[compact.result_idx][:inst] = values[1]
+                    else
+                        @assert bb_start
+                        compact.result[compact.result_idx][:inst] = PhiNode(edges, values)
+                    end
                 else
                     ri = process_node!(compact, compact.result_idx, node, i, i, ms, true)
                     if ri == compact.result_idx
@@ -2318,6 +2326,7 @@ function cfg_simplify!(ir::IRCode)
                 compact.result_idx += 1
             end
             ms = merged_succ[ms]
+            bb_start = false
         end
     end
     compact.idx = length(ir.stmts)