cmd/compile: reduce allocations in prove by reusing posets

In prove, reuse posets between different functions by storing them
in the per-worker cache.

Allocation count regression caused by prove improvements is down
from 5% to 3% after this CL.

Updates #25179

Change-Id: I6d14003109833d9b3ef5165fdea00aa9c9e952e8
Reviewed-on: https://go-review.googlesource.com/110455
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -152,13 +152,8 @@ type poset struct {
 	undo      []posetUndo   // undo chain
 }
 
-func newPoset(unsigned bool) *poset {
-	var flags uint8
-	if unsigned {
-		flags |= posetFlagUnsigned
-	}
+func newPoset() *poset {
 	return &poset{
-		flags:     flags,
 		values:    make(map[ID]uint32),
 		constants: make([]*Value, 0, 8),
 		nodes:     make([]posetNode, 1, 16),
@@ -168,6 +163,14 @@ func newPoset(unsigned bool) *poset {
 	}
 }
 
+func (po *poset) SetUnsigned(uns bool) {
+	if uns {
+		po.flags |= posetFlagUnsigned
+	} else {
+		po.flags &^= posetFlagUnsigned
+	}
+}
+
 // Handle children
 func (po *poset) setchl(i uint32, l posetEdge) { po.nodes[i].l = l }
 func (po *poset) setchr(i uint32, r posetEdge) { po.nodes[i].r = r }