rs6000: Give an argument to every REG_CFA_REGISTER (PR85645)

rs6000: Give an argument to every REG_CFA_REGISTER (PR85645)

The one for the prologue mflr did not have any value set, which means
use the SET that is in the insn pattern.  This works fine, except when
some late pass decides to replace the SET_SRC -- this changes the
meaning of the REG_CFA_REGISTER!  Such passes should not do these
things, but let's be more explicit here, for extra robustness.  It
could be argued that this defaulting is a design misfeature (it does
not save much space either, etc.)

	PR rtl-optimization/85645
	* config/rs6000/rs6000.c (rs6000_emit_prologue_components): Put a SET
	in the REG_CFA_REGISTER note for LR, don't leave it empty.

From-SVN: r260077