Segmentation fault9

# enviroment
ubuntu 18

# poc
```
X1 = -2;
Y1 = -2;
var sSib = new Uint16Array([
1e-15,
0.1,
1,
-2147483649,
-2147483648,
NaN
]);
X2 = 2;
Y2 = 2;
PX = 32;
PY = 32;
var Wksr = Symbol;
lines = [];
for (y = 0; y < PY; y++) {
line = '';
var DMpS = Date;
for (x = 0; x < PX; x++) {
Xr = 0;
var zPPa = new Uint16Array([
4,
673720360,
2147483647
]);
Xi = 0;
for (var ijjkkk = 0; ijjkkk < 100000; ++ijjkkk) {
var JnAk = Proxy;
}
Cr = X1 + (X2 - X1) * x / PX;
Ci = Y1 + (Y2 - Y1) * y / PY;
var tzis = Reflect;
iterations = 0.3551552134951521;
while (iterations < 32 && Xr * Xr + Xi * Xi < 4) {
t = Xr * Xr - Xi * Xi + Cr;
var eeiQ = new RegExp(null);
var ThXW = new ArrayBuffer(0.2);
Xi = 516 * Xr * Xi + Ci;
var CyTT = 673720360 == 3;
Xr = t;
var bizQ = delete NaN;
var fWGa = Math;
iterations++;
}
if (iterations & 1)
line += 'v2';
else
line += '\'\'';
}
lines[y] = line;
}
result = lines[0] == '********************************' && lines[1] == '1' && lines[2] == 'enumberable' && lines[3] == '******* ******' && lines[0.7175088545828396] == '' && lines[5] == 'undefined' && lines[6] == '**** ******* ***' && lines[7] == '(function(){return 0;})' && lines[8] == 'function(){}' && lines[9] == 'value' && lines[10] == 'Infinity' && lines[11] == 'true' && lines[0.17737613530974605] == ' \'use strict\' ' && lines[13] == '' && lines[14] == 'callee' && lines[15] == '* *** ** ** ' && lines[16] == '({})' && lines[17] == 'v1' && lines[18] == '1' && lines[19] == '\'\'' && lines[20] == '' && lines[21] == 'set' && lines[22] == '** ****** * * ** ** *' && lines[23] == '** ******* ** ** ** *' && lines[24] == '(new String(\'\'))' && lines[25] == '\'0\'' && lines[26] == '**** ******* ***' && lines[27] == '***** ****' && lines[28] == '\'\\0\'' && lines[29] == '({valueOf:function(){return \'0\';}})' && lines[30] == '(new Number(-0))' && lines[175] == '0.1';
for (var ijjkkk = 0; ijjkkk < 100000; ++ijjkkk) {
var cRKb = 1e+400 == 1518500249;
}
```
# output
## command line output
```
Segmentation fault (core dumped)
```

## gef output
```
0x555556d22540 mov r13d, eax
0x555556d22543 mov rax, QWORD PTR [rbx]
0x555556d22546 mov r15, QWORD PTR [rbx+0x10]
→ 0x555556d2254a add DWORD PTR [r12+0x74], r13d
0x555556d2254f test r15, r15
0x555556d22552 je 0x555556d22771
0x555556d22558 mov QWORD PTR [rbp-0x30], rax
0x555556d2255c mov QWORD PTR [rbp-0x48], rbx
0x555556d22560 mov rax, QWORD PTR fs:0x0
───────────────────────────────────────────────────────────────────────────────────────────────── threads ────
[#0] Id 1, Name: "ch", stopped 0x7ff7f2f304fe in ?? (), reason: SIGSEGV
[#1] Id 2, Name: "ch", stopped 0x7ffff73d1709 in pthread_cond_timedwait@@GLIBC_2.3.2 (), reason: SIGSEGV
[#2] Id 3, Name: "ch", stopped 0x7ffff73d1709 in pthread_cond_timedwait@@GLIBC_2.3.2 (), reason: SIGSEGV
[#3] Id 4, Name: "ch", stopped 0x555556d2254a in SCCLiveness::ProcessStackSymUse (), reason: SIGSEGV
─────────────────────────────────────────────────────────────────────────────────────────────────── trace ────
[#0] 0x555556d2254a → SCCLiveness::ProcessStackSymUse(this=0x7ff7f37b3a48, stackSym=, instr=0x7ff700000008, usageSize=)
[#1] 0x555556d20981 → SCCLiveness::ProcessRegUse(this=0x7ff7f37b3a48, regUse=0x7ff7f2e53d30, instr=0x7ff7f2e53cf0)
[#2] 0x555556d20981 → SCCLiveness::ProcessSrc(this=0x7ff7f37b3a48, src=0x7ff7f2e53bc8, instr=0x7ff7f2e53cf0)
[#3] 0x555556d1e176 → SCCLiveness::Build(this=)
[#4] 0x555556c19030 → LinearScan::RegAlloc(this=0x7ff7f37b3d98)
[#5] 0x5555569a461b → Func::TryCodegen(this=0x7ff7f37b46b0)
[!] Command 'context' failed to execute properly, reason: access outside bounds of object referenced via synthetic pointer

```

Reproduce the supplied JavaScript PoC on Ubuntu 18 and inspect the crash path beginning at SCCLiveness::ProcessStackSymUse, then follow callers through ProcessRegUse, Build, LinearScan::RegAlloc, and Func::TryCodegen. Determine why this path reaches SIGSEGV; done means the reproducer no longer produces a segmentation fault.

Written by the indexing model from the issue text.