Remove dead code in instcombine.

Don't replace shifts greater than the type with the maximum shift.

This isn't hit anywhere in the tests, and somewhere else is replacing
these with undef.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207000 91177308-0d34-0410-b5e6-96231b3b80d8