Optimize store of "bitcast" from vector to aggregate.

This patch is what was the "instcombine" portion of D14185, with an additional 
test added (see julia_pseudovec in test/Transforms/InstCombine/insert-val-extract-elem.ll). 
The patch causes instcombine to replace sequences of extractelement-insertvalue-store 
that act essentially like a bitcast followed by a store.

Differential review: http://reviews.llvm.org/D14260

llvm-svn: 267482