Fix g++ build (Add extra scope block for switch case)

[bram-perl]

Code in 9fdd7fc broke the g++ builds,

Basically after the commit the code looked like:

     switch (o->op_type) {
     ...
     case OP_SASSIGN:
         ...
         OP* rhs = cBINOPx(o)->op_first;
         OP* lval = cBINOPx(o)->op_last;
         ...
         break;

     case OP_AASSIGN: {

g++ does not allow this and errors with:

peep.c:3897:14: error: jump to case label
 3897 |         case OP_AASSIGN: {
      |              ^~~~~~~~~~
peep.c:3844:17: note:   crosses initialization of 'OP* lval'
 3844 |             OP* lval = cBINOPx(o)->op_last;
      |                 ^~~~
peep.c:3843:17: note:   crosses initialization of 'OP* rhs'
 3843 |             OP* rhs = cBINOPx(o)->op_first;
      |                 ^~~

This happens because rhs and lval are not scoped in the case statement
so it could fall through to the next case.

The solution is to scope them which this commit now does by adding a
separate scope for OP_SASSIGN (similar to OP_AASSIGN).

Fixes #20108

[leonerd]

LGTM

[jkeenan]

$ gitcurr
bram/fix-gh-20108-fix-g++-build

$ git describe
v5.37.2-180-gfa3e3c8122

$ uname -mrs
FreeBSD 12.3-RELEASE-p1 amd64

$ g++ --version
g++ (FreeBSD Ports Collection) 10.3.0
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ ./perl -Ilib -V:config_args
config_args='-des -Dusedevel -Dcc=g++';

...
$ make test_harness
...
Result: PASS

LGTM