c: Fix -Wduplicated-branches ICE [PR97125]

c: Fix -Wduplicated-branches ICE [PR97125]

We crash here because since r11-3302 the C FE uses codes like SWITCH_STMT
in the else branches in the attached test, and inchash::add_expr in
do_warn_duplicated_branches doesn't handle these front-end codes.  In
the C++ FE this works because by the time we get to do_warn_duplicated_branches
we've already cp_genericize'd the SWITCH_STMT tree into a SWITCH_EXPR.

The fix is to call do_warn_duplicated_branches_r only after loops and other
structured control constructs have been lowered.

gcc/c-family/ChangeLog:

	PR c/97125
	* c-gimplify.c (c_genericize): Only call do_warn_duplicated_branches_r
	after loops and other structured control constructs have been lowered.

gcc/testsuite/ChangeLog:

	PR c/97125
	* c-c++-common/Wduplicated-branches-15.c: New test.