add fast-path to inference schanged computation

when merging two frames (such as when handling exception frames),
most of the time new is old, but the computation here is
exponential in the number of variables so we want the inner computation
here to be ultra-fast in the common case

fix #15346

base/inference.jl

@@ -1416,27 +1416,28 @@ function tmerge(typea::ANY, typeb::ANY)
 end
 
 function smerge(sa::Union{NotFound,VarState}, sb::Union{NotFound,VarState})
+    sa === sb && return sa
     sa === NF && return sb
     sb === NF && return sa
-    issubstate(sa,sb) && return sb
-    issubstate(sb,sa) && return sa
-    VarState(tmerge(sa.typ, sb.typ), sa.undef | sb.undef)
+    issubstate(sa, sb) && return sb
+    issubstate(sb, sa) && return sa
+    return VarState(tmerge(sa.typ, sb.typ), sa.undef | sb.undef)
 end
 
-tchanged(n::ANY, o::ANY) = o===NF || (n!==NF && !(n ⊑ o))
-schanged(n::ANY, o::ANY) = o===NF || (n!==NF && !issubstate(n, o))
+@inline tchanged(n::ANY, o::ANY) = o === NF || (n !== NF && !(n ⊑ o))
+@inline schanged(n::ANY, o::ANY) = (n !== o) && (o === NF || (n !== NF && !issubstate(n, o)))
 
 function stupdate!(state::Tuple{}, changes::StateUpdate)
     newst = copy(changes.state)
     if isa(changes.var, Slot)
         newst[changes.var.id] = changes.vtype
     end
-    newst
+    return newst
 end
 
 function stupdate!(state::VarTable, change::StateUpdate)
     for i = 1:length(state)
-        if isa(change.var,Slot) && i == change.var.id
+        if isa(change.var, Slot) && i == (change.var::Slot).id
             newtype = change.vtype
         else
             newtype = change.state[i]
@@ -1794,8 +1795,8 @@ function typeinf_frame(frame)
             changes = abstract_interpret(stmt, s[pc]::Array{Any,1}, frame)
             if changes === ()
                 # this line threw an error and there is no need to continue
+                #changes = s[pc]
                 break
-                changes = s[pc]
             end
             if frame.cur_hand !== ()
                 # propagate type info to exception handler
@@ -1872,16 +1873,6 @@ function typeinf_frame(frame)
                         push!(W, l)
                         s[l] = newstate
                     end
-#                        if frame.handler_at[l] === 0
-#                            frame.n_handlers += 1
-#                            if frame.n_handlers > 25
-#                                # too many exception handlers slows down inference a lot.
-#                                # for an example see test/libgit2.jl on 0.5-pre master
-#                                # around e.g. commit c072d1ce73345e153e4fddf656cda544013b1219
-#                                inference_stack = (inference_stack::CallStack).prev
-#                                return (ast0, Any, false)
-#                            end
-#                        end
                     frame.handler_at[l] = frame.cur_hand
                 elseif hd === :leave
                     for i = 1:((stmt.args[1])::Int)