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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@859 91177308-0d34-0410-b5e6-96231b3b80d8