ptr: fix offset intrinsic

This was intended to always use inbounds pointer arithmetic now.
Work around a compiler crash folding labeled break. #9129

Servo is hitting this problem, so this is a workaround for a lack of a real solution.
auto merge of #9173 : thestinger/rust/offset, r=alexcrichton

This was intended to always use inbounds pointer arithmetic now.
auto merge of #9176 : brson/rust/issue-9129, r=catamorphism

Servo is hitting this problem, so this is a workaround for lack of a real solution.

No tests because I couldn't actually reproduce the problem with either of the testcases in #9129