Add testcase for backward threading from never-executed entry edges [PR105679]

r13-1891-g49ba4fdeb648c1 made the backward threader reject paths
whose entry edge is probably never executed, but landed without a
testcase.  Add one retroactively.

[Also AI generated.]

Assisted-by: Claude Fable 5 (Anthropic)

gcc/testsuite/ChangeLog:

	PR tree-optimization/105679
	PR tree-optimization/126906
	* gcc.dg/tree-ssa/pr105679.c: New test.