[Clang] Wrong code at -O1 on x86_64-linux_gnu

Compiler Explorer: https://godbolt.org/z/38G7fYeco

- fail: trunk, 22, 21, 20, 19, 18
- work: 17.0.1 and earlier

Bisected to [d218092](https://github.com/llvm/llvm-project/commit/d218092543b3f9ba2204d7c8fe5ac70befa3d772), which is committed by @dtcxzyw.

small.c
```c
int a, d, f;
long long c[5];
int g(short h) {
long e = h & 1048575;
switch (e)
case 4:
case 6:
for (;;)
;
return 0;
}
long i(long long *h, long j) {
for (int k = 0; k < 5; k++) {
int b = h[k] ^ j;
switch ((b + a - 50) % 6u) {
case 3:
d += 5;
break;
case 0:
d = b + 7080;
d = g(b + 050) + b + j + b * 3;
case 5:
f = 1;
case 1:
d = b + b;
case 2:
case 4:
d ^= 3;
}
a = 50;
}
return d + j + d + ((char)(d + 7) + d - 3) + f;
}
int main() { i(c, -976002); }
```

Reproduce:

```
$ clang-trunk -O0 small.c && ./a.out
$ clang-trunk -O2 small.c && ./a.out
$ clang-trunk -O1 small.c && ./a.out
[1] 3335073 segmentation fault (core dumped) ./a.out
```

Start with the small.c reproducer and compare its behavior when compiled with clang-trunk at -O0, -O1, and -O2. Investigate the bisected commit d218092 and the optimization path used at -O1. Done means the reproducer no longer produces a segmentation fault with the affected compiler versions, with regression coverage added where appropriate.

Written by the indexing model from the issue text.