Fix ICE [PR target/100549]

When arg0 is same as arg1 in __builtin_ia32_pcmpgtw,
gimple_build (&stmts, GT_EXPR, cmp_type, arg0, arg1) will simplify the
comparison to vector constant 0, no stmts is generated, which causes
ICE in gsi_insert_before (gsi, stmts, GSI_SAME_STMT). So use
gsi_insert_seq_before instead which will handle NULL seq.

gcc/ChangeLog:

	PR target/100549
	* config/i386/i386.c (ix86_gimple_fold_builtin): Use
	gsi_insert_seq_before instead.

gcc/testsuite/ChangeLog:

	PR target/100549
	* gcc.target/i386/pr100549.c: New test.