Make deadargelim a bit less smart, so it doesn't choke on nested structs as

Make deadargelim a bit less smart, so it doesn't choke on nested structs as
return values that are still (partially) live. Instead of updating all uses of
a call instruction after removing some elements, it now just rebuilds the
original struct (With undef gaps where the unused values were) and leaves it to
instcombine to clean this up.

The added testcase still fails currently, but this is due to instcombine which
isn't good enough yet. I will fix that part next.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53608 91177308-0d34-0410-b5e6-96231b3b80d8