reflect: allocate memory in TypeAssert[I] only when the assertion succeeds

reflect: allocate memory in TypeAssert[I] only when the assertion succeeds

goos: linux
goarch: amd64
pkg: reflect
cpu: 11th Gen Intel(R) Core(TM) i7-1185G7 @ 3.00GHz
                                                                        │ /tmp/before │              /tmp/after               │
                                                                        │   sec/op    │   sec/op     vs base                  │
TypeAssert/TypeAssert[int](int)-8                                         2.599n ± 1%   2.558n ± 0%  -1.56% (p=0.000 n=30)
TypeAssert/TypeAssert[uint8](int)-8                                       2.506n ± 1%   2.579n ± 2%  +2.93% (p=0.008 n=30)
TypeAssert/TypeAssert[fmt.Stringer](reflect_test.testTypeWithMethod)-8    7.449n ± 2%   7.776n ± 2%  +4.39% (p=0.000 n=30)
TypeAssert/TypeAssert[fmt.Stringer](*reflect_test.testTypeWithMethod)-8   7.220n ± 2%   7.439n ± 1%  +3.04% (p=0.000 n=30)
TypeAssert/TypeAssert[interface_{}](int)-8                                4.015n ± 1%   4.207n ± 1%  +4.79% (p=0.000 n=30)
TypeAssert/TypeAssert[interface_{}](reflect_test.testTypeWithMethod)-8    4.003n ± 1%   4.190n ± 2%  +4.66% (p=0.000 n=30)
TypeAssert/TypeAssert[time.Time](time.Time)-8                             2.933n ± 1%   2.942n ± 1%       ~ (p=0.327 n=20+30)
TypeAssert/TypeAssert[func()_string](func()_string)-8                                   111.5n ± 1%
geomean                                                                   4.004n        6.208n       +2.62%

Change-Id: I6a6a6964d6f9c794adc15dc5ff3dc8634b30df89
Reviewed-on: https://go-review.googlesource.com/c/go/+/705255
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>