ScalarEvolution: Add URem support

In LLVM IR the following code:

    %r = urem <ty> %t, %b

is equivalent to:

    %q = udiv <ty> %t, %b
    %s = mul <ty> nuw %q, %b
    %r = sub <ty> nuw %t, %q ; (t / b) * b + (t % b) = t

As UDiv, Mul and Sub are already supported by SCEV, URem can be
implemented with minimal effort this way.

Note: While SRem and SDiv are also related this way, SCEV does not
provides SDiv yet.

llvm-svn: 306695