cmd/compile: adding rule to eliminate ANDCCconst

For example, the Slicemask rule in PPC64 generates a sequence wherein there is andi operation, after an  sradi, which can be replaced by srdi. This new rule eliminates ANDCCconst.

Change-Id: I27aaadf76b9c749a60bcdc5e87b1ebb8167d2fd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/539055
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Jayanth Krishnamurthy <jayanth.krishnamurthy@ibm.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

Author: jkrishmys

src/cmd/compile/internal/ssa/_gen/PPC64.rules

@@ -800,6 +800,7 @@
 (AtomicOr(8|32)   ...) => (LoweredAtomicOr(8|32)   ...)
 
 (Slicemask <t> x) => (SRADconst (NEG <t> x) [63])
+(Select0 (ANDCCconst [1] z:(SRADconst [63] x))) && z.Uses == 1  => (SRDconst [63] x)
 
 // Note that MOV??reg returns a 64-bit int, x is not necessarily that wide
 // This may interact with other patterns in the future. (Compare with arm64)