Only use instructions as insert locations for SCEVExpander

SCEVExpander, which we are using during code generation, only allows
instructions as insert locations, but breaks in case BasicBlock->end() iterators
are passed to it due to it trying to obtain the basic block in which code should
be generated by calling Instruction->getParent(), which is not defined for
->end() iterators.

This change adds an assert to Polly that ensures we only pass valid instructions
to SCEVExpander and it fixes one case, where we used IRBuilder->SetInsertBlock()
to set an ->end() insert location which was later passed to SCEVExpander.

In general, Polly is always trying to build up the CFG first, before we actually
insert instructions into the CFG sceleton. As a result, each basic block should
already have at least one branch instruction before we start adding code. Hence,
always requiring the IRBuilder insert location to be set to a real instruction
should always be possible.

Thanks Utpal Bora <cs14mtech11017@iith.ac.in> for his help with test case
reduction.

llvm-svn: 243830