Allow AVX vrsqrtps generation.

This is a follow-on to r220570 that allows a 256-bit (v8f32)
version of vrsqrtps to be generated.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220579 91177308-0d34-0410-b5e6-96231b3b80d8