Fix a nasty bug that caused us to unroll EXTREMELY large loops due to overflow

Fix a nasty bug that caused us to unroll EXTREMELY large loops due to overflow
in the size calculation.

This is not something you want to see:
Loop Unroll: F[main] Loop %no_exit Loop Size = 2 Trip Count = 2147483648 - UNROLLING!

The problem was that 2*2147483648 == 0.

Now we get:
Loop Unroll: F[main] Loop %no_exit Loop Size = 2 Trip Count = 2147483648 - TOO LARGE: 4294967296>100

Thanks to some anonymous person playing with the demo page that repeatedly
caused zion to go into swapping land.  That's one way to ensure you'll get
a quick bugfix.  :)

Testcase here: Transforms/LoopUnroll/2004-05-13-DontUnrollTooMuch.ll

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13564 91177308-0d34-0410-b5e6-96231b3b80d8