[release-branch.go1.13] cmd/compile: fix deallocation of live value copies in regalloc
    
    When deallocating the input register to a phi so that the phi
    itself could be allocated to that register the code was also
    deallocating all copies of that phi input value. Those copies
    of the value could still be live and if they were the register
    allocator could reuse them incorrectly to hold speculative
    copies of other phi inputs. This causes strange bugs.
    
    No test because this is a very obscure scenario that is hard
    to replicate but CL 228060 adds an assertion to the compiler
    that does trigger when running the std tests on linux/s390x
    without this CL applied. Hopefully that assertion will prevent
    future regressions.
    
    Fixes #38442.
    
    Change-Id: Id975dadedd731c7bb21933b9ea6b17daaa5c9e1d
    Reviewed-on: https://go-review.googlesource.com/c/go/+/228061
    Run-TryBot: Michael Munday <mike.munday@ibm.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Keith Randall <khr@golang.org>
    (cherry picked from commit 382fe3e2498f2066400e7e7007aa9903440e339d)
    Reviewed-on: https://go-review.googlesource.com/c/go/+/230358