cmd/compile: remove Value.RemoveArg

It's only used in two places:

 - The one in regalloc.go can be replaced with v.resetArgs()
 - The one in rewrite.go can be open coded

and can cause wrong usage like the bug that CL 358117 fixed.

Change-Id: I125baf237db159d056fe4b1c73072331eea4d06a
Reviewed-on: https://go-review.googlesource.com/c/go/+/357965
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -1840,7 +1840,7 @@ func (s *regAllocState) regalloc(f *Func) {
 				if s.f.pass.debug > regDebug {
 					fmt.Printf("delete copied value %s\n", c.LongString())
 				}
-				c.RemoveArg(0)
+				c.resetArgs()
 				f.freeValue(c)
 				delete(s.copies, c)
 				progress = true

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

@@ -816,7 +816,11 @@ func devirtLECall(v *Value, sym *obj.LSym) *Value {
 	v.Op = OpStaticLECall
 	auxcall := v.Aux.(*AuxCall)
 	auxcall.Fn = sym
-	v.RemoveArg(0)
+	// Remove first arg
+	v.Args[0].Uses--
+	copy(v.Args[0:], v.Args[1:])
+	v.Args[len(v.Args)-1] = nil // aid GC
+	v.Args = v.Args[:len(v.Args)-1]
 	return v
 }
 

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

@@ -302,12 +302,6 @@ func (v *Value) SetArg(i int, w *Value) {
 	v.Args[i] = w
 	w.Uses++
 }
-func (v *Value) RemoveArg(i int) {
-	v.Args[i].Uses--
-	copy(v.Args[i:], v.Args[i+1:])
-	v.Args[len(v.Args)-1] = nil // aid GC
-	v.Args = v.Args[:len(v.Args)-1]
-}
 func (v *Value) SetArgs1(a *Value) {
 	v.resetArgs()
 	v.AddArg(a)