gccrs: Fix overflow intrinsic use before init

gccrs: Fix overflow intrinsic use before init

The overflow intrinsic returns a tuple of (value, boolean) where it value
is the operator result and boolean if it overflowed or not. The intrinsic
here did not initilize the resulting tuple and therefore was creating a use
before init error resulting in garbage results

Addresses #1895

gcc/rust/ChangeLog:

	* backend/rust-compile-intrinsic.cc (op_with_overflow_inner): fix use before init

Signed-off-by: Philip Herron <herron.philip@googlemail.com>