PR116510: Add missing fold_converts into tree switch if conversion

Passes test suite. Ok to commit?

gcc/ChangeLog:

	PR middle-end/116510
	* tree-if-conv.cc (predicate_bbs): Add missing fold_converts.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-switch-ifcvt-3.c: New test.