Fix float->int conversion warnings when near barriers.

Fix float->int conversion warnings when near barriers.

As Eli brought up here: https://reviews.llvm.org/D46535
I'd previously messed up this fix by missing conversions
that are just slightly outside the range.  This patch fixes
this by no longer ignoring the return value of
convertToInteger.  Additionally, one of the error messages
wasn't very sensical (mentioning out of range value, when it
really was not), so it was cleaned up as well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@331812 91177308-0d34-0410-b5e6-96231b3b80d8