[X86][AVX] Access a scalar float/double as a free extract from a broadcast load ...

[X86][AVX] Access a scalar float/double as a free extract from a broadcast load (PR43217)

If a fp scalar is loaded and then used as both a scalar and a vector broadcast, perform the load as a broadcast and then extract the scalar for 'free' from the 0th element.

This involved switching the order of the X86ISD::BROADCAST combines so we only convert to X86ISD::BROADCAST_LOAD once all other canonicalizations have been attempted.

Adds a DAGCombinerInfo::recursivelyDeleteUnusedNodes wrapper.

Fixes PR43217

Differential Revision: https://reviews.llvm.org/D68544

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373871 91177308-0d34-0410-b5e6-96231b3b80d8