Tail call optimization improvements:

Move platform independent code (lowering of possibly overwritten
arguments, check for tail call optimization eligibility) from
target X86ISelectionLowering.cpp to TargetLowering.h and
SelectionDAGISel.cpp.

Initial PowerPC tail call implementation:

Support ppc32 implemented and tested (passes my tests and
test-suite llvm-test).  
Support ppc64 implemented and half tested (passes my tests).
On ppc tail call optimization is performed if 
  caller and callee are fastcc
  call is a tail call (in tail call position, call followed by ret)
  no variable argument lists or byval arguments
  option -tailcallopt is enabled
Supported:
 * non pic tail calls on linux/darwin
 * module-local tail calls on linux(PIC/GOT)/darwin(PIC)
 * inter-module tail calls on darwin(PIC)
If constraints are not met a normal call will be emitted.

A test checking the argument lowering behaviour on x86-64 was added.

llvm-svn: 50477

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