Merging r339515:

------------------------------------------------------------------------
r339515 | d0k | 2018-08-12 13:43:03 +0200 (Sun, 12 Aug 2018) | 4 lines

[InstSimplify] Guard against large shift amounts.

These are always UB, but can happen for large integer inputs. Testing it
is very fragile as -simplifycfg will nuke the UB top-down.
------------------------------------------------------------------------


git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_70@339985 91177308-0d34-0410-b5e6-96231b3b80d8