middle-end/107679 - fix SSA rewrite of clobber of parameter

When the frontend clobbers a parameter and that parameter gets
rewritten into SSA then we ICE because we didn't expect this.  Avoid
using the parameter decl to create a SSA default def in this case.

	PR middle-end/107679
	* tree-into-ssa.cc (maybe_register_def): Use new temporary if
	we require an uninitialized value for a parameter decl.

	* gfortran.dg/pr107679.f90: New testcase.