cmd/compile/internal/ssa: re-adjust CarryChainTail scheduling priority

This needs to be as low as possible while not breaking priority
assumptions of other scores to correctly schedule carry chains.

Prior to the arm64 changes, it was set below ReadTuple. At the time,
this prevented the MulHiLo implementation on PPC64 from occluding
the scheduling of a full carry chain.

Memory scores can also prevent better scheduling, as can be observed
with crypto/internal/edwards25519/field.feMulGeneric.

Fixes #56497

Change-Id: Ia4b54e6dffcce584faf46b1b8d7cea18a3913887
Reviewed-on: https://go-review.googlesource.com/c/go/+/447435
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

0 files changed, 0 insertions, 0 deletions