x86: Fix up ssse3_pshufbv8qi splitter

x86: Fix up ssse3_pshufbv8qi splitter

The constant pool size optimization I was testing resulted in various ICEs
in gcc.target/i386/ testsuite, the problem is that the ssse3_pshufbv8qi
splitter emits invalid RTL, in V4SImode 0xf7f7f7f7 CONST_INTs shouldn't
appear, instead they should have been -0x8080809 (0xf7f7f7f7 sign extended
into 64 bits).

2020-08-30  Jakub Jelinek  <jakub@redhat.com>

	* config/i386/sse.md (ssse3_pshufbv8qi): Use gen_int_mode instead of
	GEN_INT, and ix86_build_const_vector instead of gen_rtvec and
	gen_rtx_CONT_VECTOR.