[llvm] r233881 - Fix a bug indicated by -fsanitize=shift-exponent.

[Sanjoy Das]

I'm having a hard time reproducing the regression locally. I
bootstrapped clang before and after r233881 with -flto and Release,
and both of them give me the same IR for the sieve benchmark (I'm on
Darwin and x86_64).  Is it possible to get some information out of the
buildbot that'll help in discovering what was actually run?

-- Sanjoy