[GlobalISel] Add G_SBFX + G_UBFX (bitfield extraction opcodes)

There is a bunch of similar bitfield extraction code throughout *ISelDAGToDAG.

E.g, ARMISelDAGToDAG, AArch64ISelDAGToDAG, and AMDGPUISelDAGToDAG all contain
code that matches a bitfield extract from an and + right shift.

Rather than duplicating code in the same way, this adds two opcodes:

- G_UBFX (unsigned bitfield extract)
- G_SBFX (signed bitfield extract)

They work like this

```
%x = G_UBFX %y, %lsb, %width
```

Where `lsb` and `width` are

- The least-significant bit of the extraction
- The width of the extraction

This will extract `width` bits from `%y`, starting at `lsb`. G_UBFX zero-extends
the result, while G_SBFX sign-extends the result.

This should allow us to use the combiner to match the bitfield extraction
patterns rather than duplicating pattern-matching code in each target.

Differential Revision: https://reviews.llvm.org/D98464

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