[CodeGen] Iterate over explicit instruction list for block statements. NFC

For when statements do not contain all instructions of a BasicBlock
anymore, the block generator needs to go through the explicit list of
instructions it contains.

Contributed-by: Nandini Singhal <cs15mtech01004@iith.ac.in>

Differential Revision: https://reviews.llvm.org/D33653

llvm-svn: 304502