instcombine: Don't replace all uses for instructions with no uses

instcombine: Don't replace all uses for instructions with no uses

My commit to migrate the printf simplifiers from the simplify-libcalls
in r168604 introduced a regression reported by Duncan [1].  The problem
is that in some cases the library call simplifier can return a new value
that has no uses and the new value's type is different than the old value's
type (which is fine because there are no uses).  The specific case that
triggered the bug looked something like:

   declare void @printf(i8*, ...)
   ...
   call void (i8*, ...)* @printf(i8* %fmt)

Which we want to optimized into:

   call i32 @putchar(i32 104)

However, the code was attempting to replace all uses of the printf with
the putchar and the types differ, hence a crash.  This is fixed by *just*
deleting the original instruction when there are no uses.  The old
simplify-libcalls pass is already doing something similar.

[1] http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-November/056338.html

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@168716 91177308-0d34-0410-b5e6-96231b3b80d8