Fix a type mismatch assert in SCEV division

An assert was triggered when attempting to create a new SCEV
with operands of different types in the visitAddRecExpr. In this
test case, the operand types of the numerator and denominator
are different. The SCEV division code should generate a
conservative answer when this happens.

Differential Revision: http://reviews.llvm.org/D9021

llvm-svn: 235511