This patch allows vectorization when operators are available as 
libfuncs, rather that only as insns.

This will be useful for amdgcn where we plan to vectorize loops that 
contain integer division or modulus, but don't want to generate inline 
instructions for the division algorithm every time.

The change should have not affect architectures that do not define 
vector-mode libfuncs.

OK for mainline?

Andrew