[X86] Improve AVX1 shuffle lowering for v8f32 shuffles where the low half...

[X86] Improve AVX1 shuffle lowering for v8f32 shuffles where the low half comes from V1 and the high half comes from V2 and the halves do the same operation

To lower this we now create a new V1 containing the low half of both sources and a new V2 containing the upper half of both sources. Then we created a repeated lane shuffle of those new sources to create the final result.

This fixes PR35833

Differential Revison: https://reviews.llvm.org/D41794

llvm-svn: 339818