[dev.regabi] cmd/compile: refactor redundant type conversion [generated]

Passes toolstash -cmp.

[git-generate]

cd src/cmd/compile
rf '
    ex . '"$(printf '%s\n' ./internal/* | paste -sd' ')"' {
        type T interface{}
        var t T
        strict t
        t.(T) -> t
    }
'
cd internal/ir
go generate

Change-Id: I492d50390e724a7216c3cd8b49d4aaf7d0c335da
Reviewed-on: https://go-review.googlesource.com/c/go/+/280716


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>