X86: silence sign comparison warning

GCC 4.8 detected a signed compare [-Wsign-compare].  Add a cast for the
destination index.  Add an assert to catch a potential overflow however unlikely
it may be.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213878 91177308-0d34-0410-b5e6-96231b3b80d8