[X86][SSE] Enable initial support for domain crossing at high shuffle combine depths.

[X86][SSE] Enable initial support for domain crossing at high shuffle combine depths.

As discussed on D27692, this permits another domain to be used to combine a shuffle at high depths.

We currently set the required depth at 4 or more combined shuffles, this is probably too high for most targets but is a good starting point and already helps avoid a number of costly variable shuffles.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295608 91177308-0d34-0410-b5e6-96231b3b80d8