[dev.simd] cmd/compile: more support for cpufeatures

add hasFeature, also record maximum feature for a function

Change-Id: I68dd063aad1c1dc0ef5310a9f5d970c03dd31a0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/710695
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/cmd/compile/internal/ssa/block.go

@@ -473,6 +473,10 @@ const (
 	CPUsve2
 )
 
+func (f CPUfeatures) hasFeature(x CPUfeatures) bool {
+	return f&x == x
+}
+
 func (f CPUfeatures) String() string {
 	if f == CPUNone {
 		return "none"

src/cmd/compile/internal/ssa/cpufeatures.go

@@ -201,6 +201,7 @@ func cpufeatures(f *Func) {
 		}
 
 		b.CPUfeatures = feat
+		f.maxCPUFeatures |= feat // not necessary to refine this estimate below
 	}
 
 	// If the flow graph is irreducible, things can still change on backedges.

src/cmd/compile/internal/ssa/func.go

@@ -41,6 +41,8 @@ type Func struct {
 	ABISelf        *abi.ABIConfig // ABI for function being compiled
 	ABIDefault     *abi.ABIConfig // ABI for rtcall and other no-parsed-signature/pragma functions.
 
+	maxCPUFeatures CPUfeatures // union of all the CPU features in all the blocks.
+
 	scheduled   bool  // Values in Blocks are in final order
 	laidout     bool  // Blocks are ordered
 	NoSplit     bool  // true if function is marked as nosplit.  Used by schedule check pass.