mark bounds check failures as the cold path

LLVM is unable to determine this for most cases.

http://llvm-reviews.chandlerc.com/D2034 needs to land upstream before
this is going to have an effect. It's harmless to start generating the
expect hint now.
auto merge of #10113 : thestinger/rust/expect, r=cmr

LLVM is unable to determine this for most cases.

http://llvm-reviews.chandlerc.com/D2034 needs to land upstream before
this is going to have an effect. It's harmless to start generating the
expect hint now.