[llvm] [ARMISelDAGToDAG] use MO_FrameIndex to represent FrameIndex rather than MO_Register (PR #69654)

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> Particularly for the constraints that are not "m"; "Uv" is explicitly documented to be an operand for a "vldr", which only has a 10 bit offset.

Yeah, I was waffling about splitting the cases in `ARMDAGToDAGISel::SelectInlineAsmMemoryOperand` to just do this change for the non-`Ux` constraints.  I could change it back.

> And it looks like the handling for the case where the the stack frame is actually larger than 4096 bytes is missing?

Is that possible to know (the current stack depth) during SelectionDAGISel.  Or should that be handled elsewhere later? GCC just errors once the stack gets too large.


https://github.com/llvm/llvm-project/pull/69654