[X86] Enable custom splitting of v8i64/v16i32 sext/zext for avx/avx2 when input type...

[X86] Enable custom splitting of v8i64/v16i32 sext/zext for avx/avx2 when input type will be promoted by the type legalize to 128-bits.

If the the input type will be promoted to 128 bits its better to put a sign_extend_inreg/and in the 128 bit register before the split occurs. Otherwise we end up doing it on each half in the wider register.

Some of the overflow arithmetic tests are regressions, but I think we can make some improvement using getSetccResultType in DAG combine and/or type legalization.

llvm-svn: 354709