[LIR] General refactor to improve compile-time and simplify code.

First create a list of candidates, then transform.  This simplifies the code in
that you have don't have to worry that you may be using an invalidated
iterator.

Previously, each time we created a memset/memcpy we would reevaluate the entire
loop potentially resulting in lots of redundant work for large basic blocks.

llvm-svn: 252817