[release-branch.go1.15] cmd/compile: don't short-circuit copies whose source is volatile
    
    Current optimization: When we copy a->b and then b->c, we might as well
    copy a->c instead of b->c (then b might be dead and go away).
    
    *Except* if a is a volatile location (might be clobbered by a call).
    In that case, we really do want to copy a immediately, because there
    might be a call before we can do the a->c copy.
    
    User calls can't happen in between, because the rule matches up the
    memory states. But calls inserted for memory barriers, particularly
    runtime.typedmemmove, can.
    
    (I guess we could introduce a register-calling-convention version
    of runtime.typedmemmove, but that seems a bigger change than this one.)
    
    Fixes #43575
    
    Change-Id: Ifa518bb1a6f3a8dd46c352d4fd54ea9713b3eb1a
    Reviewed-on: https://go-review.googlesource.com/c/go/+/282492
    Trust: Keith Randall <khr@golang.org>
    Trust: Josh Bleecher Snyder <josharian@gmail.com>
    Run-TryBot: Keith Randall <khr@golang.org>
    TryBot-Result: Go Bot <gobot@golang.org>
    Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
    (cherry picked from commit 304f769ffc68e64244266b3aadbf91e6738c0064)
    Reviewed-on: https://go-review.googlesource.com/c/go/+/282558
    Trust: Dmitri Shuralyov <dmitshur@golang.org>