aarch64: Don't fold svundef* at the gimple level

aarch64: Don't fold svundef* at the gimple level

As the testcase shows, folding svundef*() at the gimple level
has the unfortunate side-effect of introducing -Wuninitialized
or -Wmaybe-uninitialized warnings.  We don't have a testcase
that relies on the fold, so the easiest fix seems to be to
remove it.

gcc/
	* config/aarch64/aarch64-sve-builtins-base.cc (svundef_impl::fold):
	Delete.

gcc/testsuite/
	* gcc.target/aarch64/sve/acle/general/undef_1.c: New test.