fix: handle zero-sized memory intrinsics

[mizoz]

Fixes #516.

This avoids lowering zero-sized copy, copy_nonoverlapping, and write_bytes intrinsics to GCC memory builtins. For zero-sized element types, the operation has no bytes to touch, so the backend can return before the fallback lowering emits memcpy, memmove, or memset.

A small run test covers the three zero-sized memory intrinsic paths using () pointers.

Tested with:

• git diff --check
• cargo fmt --check
• ./y.sh test --mini-tests
• cargo test -p rustc_codegen_gcc --test lang_tests zero_sized_mem -- --nocapture

[bjorn3]

Why not change the memset, memcpy and memmove methods on Builder? Also this only helps with ZST types, not with copying zero elements.

View changes since the review

[mizoz]

You're right — this should cover the generic memcpy/memmove/memset paths and not only the ZST intrinsic case.

I had avoided changing the builder methods because a first pass that split the current block inside those methods caused later MIR emission to keep writing to the terminated block in local testing. I'm going to rework this instead of trying to paper over it, so the updated patch handles runtime zero sizes cleanly as well.