Fix an assertion when the compiler encounters big constants whose bit width is
not a multiple of 64-bits.
Although clang would never generate something like this, the backend should be
able to handle any legal IR.
<rdar://problem/13363576>
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183544 91177308-0d34-0410-b5e6-96231b3b80d8
