Avoid disabling dead code elimination when using Equal

[pgimalac]

Partially fixes https://github.com/google/go-cmp/issues/373 (Equal only).

This PR moves code around to allow the linker to statically determine that the code using reflect.Type.Method is not reachable from the Equal function. It replaces the type name logic of unnamed interfaces to just use reflect.Type.String() when the qualified argument of appendTypeName is false. The only difference in the output is that there is a space added between interface and the opening bracket.

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