[LV] Fix for PR37248, Broadcast codegen incorrectly assumed vector loop body is single basic block

Summary:
Broadcast code generation emitted instructions in pre-header, while the instruction they are dependent on in the vector loop body.
This resulted in an IL verification error ---- value used before defined.


Reviewers: rengolin, fhahn, hfinkel

Reviewed By: rengolin, fhahn

Subscribers: dcaballe, Ka-Ka, llvm-commits

Differential Revision: https://reviews.llvm.org/D46302

llvm-svn: 331799