Fixed a bug where we did not handle constant

expressions correctly.  These produced a result
variable with an initializer but no store
instruction, and the store instruction was as
a result never rewritten to become a store to a
persistent variable.

Now if the result variable has an initializer
but is never used, we generate a (redundant)
store instruction for it, which is then later
rewritten into a (useful) store to the persistent
result variable.

llvm-svn: 113300