_mjit_compile_pc_and_sp.erb: make sure no uninitialized

area on VM stack to prevent SEGV on GC. GC may mark every value in VM
stack.

Unfortunately I couldn't write a test for it... So let me explain the
situation.

SEGV example:
https://gist.github.com/k0kubun/c7cea2b5761ffdff29ec79ea1a8f7f91

```
$ ruby --dump=insns -e 'def oct(num, len); "%0#{len}o" % num; end'
== disasm: #<ISeq:oct@-e:1 (1,0)-(1,41)> (catch: FALSE)
local table (size: 2, argc: 2 [opts: 0, rest: -1, post: 0, block: -1, kw: -1@-1, kwrest: -1])
[ 2] num@0<Arg> [ 1] len@1<Arg>
0000 putobject                    "%0"                                (   1)[LiCa]
0002 getlocal_WC_0                len@1
0004 dup
0005 checktype                    T_STRING
0007 branchif                     14
0009 dup
0010 opt_send_without_block       <callinfo!mid:to_s, argc:0, FCALL|ARGS_SIMPLE>, <callcache>
0013 tostring
0014 putobject                    "o"
0016 concatstrings                3
0018 getlocal_WC_0                num@0
0020 opt_mod                      <callinfo!mid:%, argc:1, ARGS_SIMPLE>, <callcache>
0023 leave                        [Re]
```

Prior to this commit, after arguments are pushed on 0010, stacks were:
VM stack: [uninitialized, uninitialized, len]
JIT stack: ["%0", len, len]

And then, when GC is invoked on 0016, VM stack will be [uninitialized, uninitialized]
and those uninitialized values will be marked by GC.

With this commit, after arguments are pushed on 0010, stacks will be:
VM stack: [len]
JIT stack: ["%0", len, len]

And VM stack will be [] on 0016.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63360 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/ruby_vm/views/_mjit_compile_pc_and_sp.erb

@@ -20,12 +20,11 @@
 % # sp motion is optimized away for `handles_frame? #=> false` case.
 % # Thus sp should be set properly before `goto cancel`.
 % if insn.handles_frame?
-            fprintf(f, "    reg_cfp->sp = (VALUE *)reg_cfp->bp + stack_size + 1 - <%= insn.pops.size %>;\n"); /* POPN(INSN_ATTR(popn)); */
-%
 %           # JIT-only behavior (pushing JIT's local variables to VM's stack):
             {
                 rb_snum_t i, push_size;
                 push_size = -<%= insn.call_attribute('sp_inc') %> + <%= insn.rets.size %> - <%= insn.pops.size %>;
+                fprintf(f, "    reg_cfp->sp = (VALUE *)reg_cfp->bp + %ld + 1;\n", push_size); /* POPN(INSN_ATTR(popn)); */
                 for (i = 0; i < push_size; i++) { /* TODO: use memcpy? */
                     fprintf(f, "    *(reg_cfp->sp + %ld) = stack[%ld];\n", i - push_size, (rb_snum_t)b->stack_size - push_size + i);
                 }