cmd/compile: remove pointer temporaries in walkcompare

cmd/compile: remove pointer temporaries in walkcompare

When comparing two T-typed values t1 and t2 using the T_eq function,
we used to generate:

    pl := &t1
    pr := &t2
    return T_eq(pl, pr, unsafe.Sizeof(T{}))

This CL changes it to simply generate:

    return T_eq(&t1, &t2, unsafe.Sizeof(T{}))

Surprisingly, this does not pass toolstash. For some reason, it seems
like SSA wasn't able to SSA-ify the pl and pr variables in all cases.

Change-Id: I111fbb068a1741fa169c9922cb8cdb6e21579aa4
Reviewed-on: https://go-review.googlesource.com/c/go/+/197601
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>