Have GVN simplify instructions as it goes. For example, consider

Have GVN simplify instructions as it goes.  For example, consider
"%z = %x and %y".  If GVN can prove that %y equals %x, then it turns
this into "%z = %x and %x".  With the new code, %z will be replaced
with %x everywhere (and then deleted).  Previously %z would be value
numbered too, which is a waste of time.  Also, while a clever value
numbering algorithm would give %z the same value number as %x, our
current one doesn't do so (at least I don't think it does).  The new
logic has an essentially equivalent effect to what you would get if
%z was given the same value number as %x, i.e. it should make value
numbering smarter.  While there, get hold of target data once at the
start rather than a gazillion times all over the place.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@118923 91177308-0d34-0410-b5e6-96231b3b80d8