[ConstProp] Remove ConstantPropagation

As discussed in
http://lists.llvm.org/pipermail/llvm-dev/2020-July/143801.html.

Currently no users outside of unit tests.

Replace all instances in tests of -constprop with -instsimplify.
Notable changes in tests:
* vscale.ll - @llvm.sadd.sat.nxv16i8 is evaluated by instsimplify, use a fake intrinsic instead
* InsertElement.ll - insertelement undef is removed by instsimplify in @insertelement_undef
llvm/test/Transforms/ConstProp moved to llvm/test/Transforms/InstSimplify/ConstProp

Reviewed By: lattner, nikic

Differential Revision: https://reviews.llvm.org/D85159