go/ssa: make a setNumable interface

Workaround crash on go/ssa in go < 1.13.
See https://github.com/golang/go/issues/29612.

Change-Id: I32687f6ee0baaf223248d5c1631663c73cbbfc65
GitHub-Last-Rev: 6bf28bf927cff6d53bcfaf199701e5451d17305e
GitHub-Pull-Request: golang/tools#162
Reviewed-on: https://go-review.googlesource.com/c/tools/+/195477
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>

go/ssa/func.go

@@ -257,6 +257,10 @@ func (f *Function) createSyntacticParams(recv *ast.FieldList, functype *ast.Func
 	}
 }
 
+type setNumable interface {
+	setNum(int)
+}
+
 // numberRegisters assigns numbers to all SSA registers
 // (value-defining Instructions) in f, to aid debugging.
 // (Non-Instruction Values are named at construction.)
@@ -267,9 +271,7 @@ func numberRegisters(f *Function) {
 		for _, instr := range b.Instrs {
 			switch instr.(type) {
 			case Value:
-				instr.(interface {
-					setNum(int)
-				}).setNum(v)
+				instr.(setNumable).setNum(v)
 				v++
 			}
 		}