Extensive changes to the way code generation occurs for function

Extensive changes to the way code generation occurs for function
call arguments and return values:
Now all copy operations before and after a call are generated during
selection instead of during register allocation.
The values are copied to virtual registers (or to the stack), but
in the former case these operands are marked with the correct physical
registers according to the calling convention.
Although this complicates scheduling and does not work well with
live range analysis, it simplifies the machine-dependent part of
register allocation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@6465 91177308-0d34-0410-b5e6-96231b3b80d8