[SCEV] Separate out constant folding in mul expr creation

Separate out the code handling constant folding into a separate
block, that is independent of other folds that need a constant
first operand. Also make some minor adjustments to make the
constant folding look nearly identical to the same code in
getAddExpr().

The only reason this change is not strictly NFC is that the
C1*(C2+V) fold is moved below the constant folding, which means
that it now also applies to C1*C2*(C3+V), as it should.