Start of a series of patches related to implicit_def.

Start of a series of patches related to implicit_def.
There is no point in creating a long live range defined by an implicit_def. Scheduler now duplicates implicit_def instruction for each of its uses. Therefore, if an implicit_def node has multiple uses, it will become a number of very short live ranges, rather than a long one. This will make coalescer's job easier.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@49164 91177308-0d34-0410-b5e6-96231b3b80d8