7026 – 64 bit optimizer bug

The following code (reduced varint encoding) works as expected on Linux x86_64 if built without flags or just one of -O and -release, but fails if both -O -release are specified:
---
import core.stdc.stdio;
ubyte foo(uint n) {
 ubyte[5] buf = void;
 ubyte wsize;
 while (true) {
 if ((n & ~0x7F) == 0) {
 buf[wsize++] = cast(ubyte)n;
 break;
 } else {
 buf[wsize++] = cast(ubyte)((n & 0x7F) | 0x80);
 n >>= 7;
 }
 }
 printf("%hhu\n", wsize);
 return buf[0];
}
void main() {
 printf("%hhx\n", foo(3));
}
---
More specifically, the output (printf()s for shorter assembly) is »1 e0« for the optimized build instead of »1 3«, and the program crashes most of the time (different errors: segfaults, illegal instruction, glibc free() assert triggers, ...) – stack corruption?

https://github.com/D-Programming-Language/dmd/commit/683624c4d7ae2f58fe2d8164ba7be77e08a4424f
https://github.com/D-Programming-Language/dmd/commit/3d21709860f771a7376eef7e7d5f924941421d02