BFI: Saturate when combining edges to a successor

When a loop gets bundled up, its outgoing edges are quite large, and can
just barely overflow 64-bits.  If one successor has multiple incoming
edges -- and that successor is getting all the incoming mass --
combining just its edges can overflow.  Handle that by saturating rather
than asserting.

This fixes PR21622.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223500 91177308-0d34-0410-b5e6-96231b3b80d8