allow @nospecialize-d push! to take arbitrary items

Currently the `@nospecialize`-d `push!(::Vector{Any}, ...)` can only
take a single item and we will end up with runtime dispatch when we try
to call it with multiple items:
```julia
julia> code_typed(push!, (Vector{Any}, Any))
1-element Vector{Any}:
 CodeInfo(
1 ─      $(Expr(:foreigncall, :(:jl_array_grow_end), Nothing, svec(Any, UInt64), 0, :(:ccall), Core.Argument(2), 0x0000000000000001, 0x0000000000000001))::Nothing
│   %2 = Base.arraylen(a)::Int64
│        Base.arrayset(true, a, item, %2)::Vector{Any}
└──      return a
) => Vector{Any}

julia> code_typed(push!, (Vector{Any}, Any, Any))
1-element Vector{Any}:
 CodeInfo(
1 ─ %1 = Base.append!(a, iter)::Vector{Any}
└──      return %1
) => Vector{Any}
```

This commit extends it so that it can take arbitrary-length items.
Our compiler should still be able to optimize the single-input case as
before by unrolling using the constant item length information:
```julia
julia> code_typed(push!, (Vector{Any}, Any))
1-element Vector{Any}:
 CodeInfo(
1 ─      $(Expr(:foreigncall, :(:jl_array_grow_end), Nothing, svec(Any, UInt64), 0, :(:ccall), Core.Argument(2), 0x0000000000000001, 0x0000000000000001))::Nothing
│   %2 = Base.arraylen(a)::Int64
│        Base.arrayset(true, a, item, %2)::Vector{Any}
└──      return a
) => Vector{Any}

julia> code_typed(push!, (Vector{Any}, Any, Any))
1-element Vector{Any}:
 CodeInfo(
1 ─ %1  = Base.arraylen(a)::Int64
│         $(Expr(:foreigncall, :(:jl_array_grow_end), Nothing, svec(Any, UInt64), 0, :(:ccall), Core.Argument(2), 0x0000000000000002, 0x0000000000000002))::Nothing
└──       goto #7 if not true
2 ┄ %4  = φ (#1 => 1, #6 => %14)::Int64
│   %5  = φ (#1 => 1, #6 => %15)::Int64
│   %6  = Base.getfield(x, %4, true)::Any
│   %7  = Base.add_int(%1, %4)::Int64
│         Base.arrayset(true, a, %6, %7)::Vector{Any}
│   %9  = (%5 === 2)::Bool
└──       goto #4 if not %9
3 ─       goto #5
4 ─ %12 = Base.add_int(%5, 1)::Int64
└──       goto #5
5 ┄ %14 = φ (#4 => %12)::Int64
│   %15 = φ (#4 => %12)::Int64
│   %16 = φ (#3 => true, #4 => false)::Bool
│   %17 = Base.not_int(%16)::Bool
└──       goto #7 if not %17
6 ─       goto #2
7 ┄       return a
) => Vector{Any}
```

base/array.jl

@@ -1053,9 +1053,13 @@ function push!(a::Array{T,1}, item) where T
     return a
 end
 
-function push!(a::Array{Any,1}, @nospecialize item)
-    _growend!(a, 1)
-    arrayset(true, a, item, length(a))
+function push!(a::Vector{Any}, @nospecialize x...)
+    na = length(a)
+    nx = length(x)
+    _growend!(a, nx)
+    for i = 1:nx
+        arrayset(true, a, x[i], na+i)
+    end
     return a
 end
 

test/compiler/inline.jl

@@ -1375,3 +1375,21 @@ let src = code_typed1() do
     @test count(isnew, src.code) == 1
     @test count(isinvoke(:noinline_finalizer), src.code) == 1
 end
+
+# optimize `push!(::Vector{Any}, x...)`
+let src = code_typed1((Vector{Any}, Any)) do xs, x
+        push!(xs, x)
+    end
+    @test count(iscall((src, push!)), src.code) == 0
+end
+let src = code_typed1((Vector{Any}, Any, Any)) do xs, x, y
+        push!(xs, x, y)
+    end
+    @test count(iscall((src, push!)), src.code) == 0
+end
+let xs = Any[]
+    push!(xs, :x, "y", 'z')
+    @test xs[1] === :x
+    @test xs[2] == "y"
+    @test xs[3] === 'z'
+end