[ARM] Speedups for CombineBaseUpdate. (#129725) This attempts to put limits onto CombineBaseUpdate for degenerate cases like #127477. The biggest change is to add a limit to the number of base updates to check in CombineBaseUpdate. 64 is hopefully plenty high enough for most runtime unrolled loops to generate postinc where they are beneficial. It also moves the check for isValidBaseUpdate later so that it only happens if we will generate a valid instruction. The 1024 limit to hasPredecessorHelper comes from the X86 backend, which uses the same limit. I haven't added a test case as it would need to be very big and my attempts at generating a smaller version did not show anything useful. Fixes #127477.
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