InstSimplify: [al]shr exact undef, %X -> undef

Exact shifts always keep the non-zero bits of their input.  This means
it keeps it's undef bits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223923 91177308-0d34-0410-b5e6-96231b3b80d8
ConstantFold: div undef, 0 should fold to undef, not zero

Dividing by zero yields an undefined value.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223924 91177308-0d34-0410-b5e6-96231b3b80d8