optimize `@memset` with `undefined`

When using `@memset` to set bytes to `undefined`, Zig notices this
case and does a single Valgrind client request rather than N.

Speeds up all allocators in safe modes.

Closes #2388
3552180143Andrew Kelley committed on 5/3/2019, 5:25:03 AMparentf8117a0
2 files changedLine totals unavailable