Move all the code that creates code generation passes from Sparc.cpp to

Move all the code that creates code generation passes from Sparc.cpp to
TargetMachine.cpp, since it is entirely machine-independent.
Also, add options to disable optional back-end passes (preselection and
instr. scheduling).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@3740 91177308-0d34-0410-b5e6-96231b3b80d8