[X86] Remove some dead code and combine some repeated code that's left.

[X86] Remove some dead code and combine some repeated code that's left.

If the width is 256 bits, then we must have AVX so the else here
was unnecessary. Once that's removed then the >= 256 bit code is
identical to the 128 bit code with a different VT so combine them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368956 91177308-0d34-0410-b5e6-96231b3b80d8