When we select the XRO variants of loads, we can pull in very specific shifts (of the size of an element). E.g.
ldr x1, [x2, x3, lsl #3]
This teaches GISel to handle these when they're coming from shifts specifically.
This adds a new addressing mode function, selectAddrModeShiftedExtendXReg which recognizes this pattern.
This also packs this up with selectAddrModeRegisterOffset into selectAddrModeXRO. This is intended to be equivalent to selectAddrModeXRO in AArch64ISelDAGtoDAG.
Also update load-addressing-modes to show that all of the cases here work.