cmd/compile: make XER allocatable register on PPC64
This is the first step towards decomposing aggregate operations
which create or consume the CA bit of the XER.
This helps optimize the canned sequence of Add64Carry (and
Sub64Borrow if it were implemented similarly) by minimizing
extraneous operations related to loading the CA bit,
reloading CA in chained operations, or extracting it when
unused.
Likewise, mark the operations which clobber CA.
Change-Id: I33e6dd2654a8cc39fcdbb9690a495f03558cdc97
Reviewed-on: https://go-review.googlesource.com/c/go/+/346869
Trust: Paul Murphy <murp@ibm.com> Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>