[ValueTracking] Out of range shifts might be undef

If it is possible for the RHS of a shift operation to be greater than or equal
to the bit-width, then the result might be undef, and we can't report any known
bits.

In some cases, this was allowing a transformation in instcombine which widened
an undef value from i1 to i32, increasing the range of values that a function
could return.

Differential revision: https://reviews.llvm.org/D30781

llvm-svn: 297724