x86: avoid maybe_gen_...()

x86: avoid maybe_gen_...()

In the (however unlikely) event that no insn can be found for the
requested mode, using maybe_gen_...() without (really) checking its
result for being a null rtx would lead to silent bad code generation.

gcc/

	* config/i386/i386-expand.cc (ix86_expand_vector_init_duplicate):
	Use gen_vec_set_0.
	(ix86_expand_vector_extract): Use gen_vec_extract_lo /
	gen_vec_extract_hi.
	(expand_vec_perm_broadcast_1): Use gen_vec_interleave_high /
	gen_vec_interleave_low. Rename local variable.