1. Add a bottom-up pass on BURG trees that is used to fix constant operands.

  Needs to be bottom up because constant values may be forward-substituted
   to their uses (i.e., into the parent in the BURG tree).
2. Move most of the constant-fixup code into machine-indepedent file
   InstrSelectionSupport.cpp.

llvm-svn: 860