[dev.ssa] cmd/compile: fix a few bugs for SSA for ARM

- 64x signed right shift was wrong for shift larger than 0x80000000.
- for Lsh-followed-by-Rsh, the intermediate value should be full int
  width, so when it is spilled MOVW should be used.
- use RET for RetJmp, so the assembler can take case of restoring LR
  for non-leaf case.
- reserve R9 in dynlink mode. R9 is used for GOT by the assembler.

Progress on SSA backend for ARM. Still not complete.

Updates #15365.

Change-Id: I3caca256b92ff7cf96469da2feaf4868a592efc5
Reviewed-on: https://go-review.googlesource.com/23793
Reviewed-by: David Chase <drchase@google.com>

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

@@ -717,7 +717,7 @@ func ssaGenBlock(s *gc.SSAGenState, b, next *ssa.Block) {
 		gc.Prog(obj.ARET)
 
 	case ssa.BlockRetJmp:
-		p := gc.Prog(obj.AJMP)
+		p := gc.Prog(obj.ARET)
 		p.To.Type = obj.TYPE_MEM
 		p.To.Name = obj.NAME_EXTERN
 		p.To.Sym = gc.Linksym(b.Aux.(*gc.Sym))