[LVI] run transfer function for binary operator even when the RHS isn't a constant

LVI was symbolically executing binary operators only when the RHS was
constant, missing the case where we have a ConstantRange for the RHS,
but not an actual constant. Tested using check-all and by
bootstrapping. Compile time is not impacted measurably.

Differential Revision: https://reviews.llvm.org/D19859

llvm-svn: 347379