[llvm] [DAG][X86] Bitfield insertion can combine into SHL + SHRD (PR #220182)

[Pratyay Pande via llvm-commits]

pratyaypande wrote:

@RKSimon, I was able to implement this in DAGCombiner. Some changes were required in `X86ISelLowering.cpp:LowerFunnelShift(...)` to expand back from the funnel shift if SHLD is slow and if we are not optimizing for code size.

I will be pushing the updated commits shortly.

https://github.com/llvm/llvm-project/pull/220182