Reapply r219832 - InstCombine: Narrow switch instructions using known bits.

The code committed in r219832 asserted when it attempted to shrink a switch
statement whose type was larger than 64-bit.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219902 91177308-0d34-0410-b5e6-96231b3b80d8