[META][X86] Remove unnecessary x86 code from DAG/X86ISelLowering

[RKSimon]

Meta ticket to track ongoing work to move unnecessary code out of x86 DAG lowering to the middle end (vectorisation etc.) or later fixup passes.

The aims of this include:

• Remove outdated codegen that the middle end no longer generates
• Better alternatives to determine optimal code for given CPU targets (TTI cost tables, MachineCombiner, X86Fixup passes etc.).
• Avoids GISel having to reimplement unnecessary codegen

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

Meta ticket to track ongoing work to move unnecessary code out of x86 DAG lowering to the middle end (vectorisation etc.) or later fixup passes.

The aims of this include:

• Remove outdated codegen that the middle end no longer generates
• Better alternatives to determine optimal code for given CPU targets (TTI cost tables, MachineCombiner, X86Fixup passes etc.).
• Avoids GISel having to reimplement unnecessary codegen