[SCEV] Make getUDivExactExpr handle non-nuw multiplies correctly.

To avoid regressions, make ScalarEvolution::createSCEV a bit more
clever.

Also get rid of some useless code in ScalarEvolution::howFarToZero
which was hiding this bug.

No new testcase because it's impossible to actually expose this bug:
we don't have any in-tree users of getUDivExactExpr besides the two
functions I just mentioned, and they both dodged the problem. I'll
try to add some interesting users in a followup.

Differential Revision: https://reviews.llvm.org/D28587

llvm-svn: 292449