inference: fix efficiency of tfunc key lookup

This was broken when `findfirst` was deprecated in 9bdf07f.

(cherry picked from commit 3aeccec)

base/compiler/optimize.jl

@@ -320,7 +320,7 @@ function statement_cost(ex::Expr, line::Int, src::CodeInfo, spvals::SimpleVector
                 atyp = argextype(ex.args[3], src, spvals, slottypes)
                 return isknowntype(atyp) ? 4 : params.inline_nonleaf_penalty
             end
-            fidx = findfirst(x->x===f, T_FFUNC_KEY)
+            fidx = find_tfunc(f)
             if fidx === nothing
                 # unknown/unhandled builtin or anonymous function
                 # Use the generic cost of a direct function call

base/compiler/tfuncs.jl

@@ -18,6 +18,13 @@ const T_IFUNC_COST = Vector{Int}(undef, N_IFUNC)
 const T_FFUNC_KEY = Vector{Any}()
 const T_FFUNC_VAL = Vector{Tuple{Int, Int, Any}}()
 const T_FFUNC_COST = Vector{Int}()
+function find_tfunc(@nospecialize f)
+    for i = 1:length(T_FFUNC_KEY)
+        if T_FFUNC_KEY[i] === f
+            return i
+        end
+    end
+end
 
 const DATATYPE_NAME_FIELDINDEX = fieldindex(DataType, :name)
 const DATATYPE_PARAMETERS_FIELDINDEX = fieldindex(DataType, :parameters)
@@ -1152,7 +1159,7 @@ function builtin_tfunction(@nospecialize(f), argtypes::Array{Any,1},
         end
         tf = T_IFUNC[iidx]
     else
-        fidx = findfirst(x->x===f, T_FFUNC_KEY)
+        fidx = find_tfunc(f)
         if fidx === nothing
             # unknown/unhandled builtin function
             return Any