Modified the check for intrinsic functions. All intrinsic functions have to

Modified the check for intrinsic functions.  All intrinsic functions have to
be at least 6 characters, since something must follow the "llvm." string in the
function name.
This seems to fix an assertion failure with the SingleSource tests, too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8418 91177308-0d34-0410-b5e6-96231b3b80d8