Revert "r293343 - [ubsan] Sanity-check shift amounts before truncation

(fixes PR27271)"

After r293343 clang fails to compile itself with -fsanitize=undefined (
http://lab.llvm.org:8080/green/job/clang-stage2-cmake-RgSan_build/).

rdar://30259929

llvm-svn: 293475