add a new -enable-value-prop flag for llcbeta, that enables propagation

add a new -enable-value-prop flag for llcbeta, that enables propagation
of value info (sign/zero ext info) from one MBB to another.  This doesn't
handle much right now because of two limitations:

1) only handles zext/sext, not random bit propagation (no assert exists
   for this)
2) doesn't handle phis.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52383 91177308-0d34-0410-b5e6-96231b3b80d8