encoding/xml: make use of reflect.TypeAssert

[apocelipes]

To make the code more readable and improve performance:

goos: darwin
goarch: arm64
pkg: encoding/xml
cpu: Apple M4
│ old │ new │
│ sec/op │ sec/op vs base │
Marshal-10 1.902µ ± 1% 1.496µ ± 1% -21.37% (p=0.000 n=10)
Unmarshal-10 3.877µ ± 1% 3.418µ ± 2% -11.84% (p=0.000 n=10)
HTMLAutoClose-10 1.314µ ± 3% 1.333µ ± 1% ~ (p=0.270 n=10)
geomean 2.132µ 1.896µ -11.07%

             │     old      │                  new                  │
             │     B/op     │     B/op      vs base                 │

Marshal-10 5.570Ki ± 0% 5.570Ki ± 0% ~ (p=1.000 n=10) ¹
Unmarshal-10 7.586Ki ± 0% 7.555Ki ± 0% -0.41% (p=0.000 n=10)
HTMLAutoClose-10 3.496Ki ± 0% 3.496Ki ± 0% ~ (p=1.000 n=10) ¹
geomean 5.286Ki 5.279Ki -0.14%
¹ all samples are equal

             │    old     │                 new                 │
             │ allocs/op  │ allocs/op   vs base                 │

Marshal-10 23.00 ± 0% 23.00 ± 0% ~ (p=1.000 n=10) ¹
Unmarshal-10 185.0 ± 0% 184.0 ± 0% -0.54% (p=0.000 n=10)
HTMLAutoClose-10 93.00 ± 0% 93.00 ± 0% ~ (p=1.000 n=10) ¹
geomean 73.42 73.28 -0.18%
¹ all samples are equal

Updates #62121

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