cmd/compile: ignore RegKill ops for non-phi after phi check

Relax the 'phi after non-phi' SSA sanity check to allow
RegKill ops interspersed with phi ops in a block. This fixes
a sanity check failure when -dwarflocationlists is enabled.

Updates #22694.

Change-Id: Iaae604ab6f1a8b150664dd120003727a6fb2f698
Reviewed-on: https://go-review.googlesource.com/77610
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -456,11 +456,16 @@ func memCheck(f *Func) {
 		for _, b := range f.Blocks {
 			seenNonPhi := false
 			for _, v := range b.Values {
-				if v.Op == OpPhi {
+				switch v.Op {
+				case OpPhi:
 					if seenNonPhi {
 						f.Fatalf("phi after non-phi @ %s: %s", b, v)
 					}
-				} else {
+				case OpRegKill:
+					if f.RegAlloc == nil {
+						f.Fatalf("RegKill seen before register allocation @ %s: %s", b, v)
+					}
+				default:
 					seenNonPhi = true
 				}
 			}