1. Remove unused `gtArrElemType` member from GenTreeArrIndex, GenTreeArrOffs
2. Add function iheader documentation for `impArrayAccessIntrinsic`
3. Add JITDUMP output reasons for rejecting MD intrinsics in `impArrayAccessIntrinsic`

No diffs

This doesn't lead to any asm diffs because it was always contained.
However, we BYREF is the correct type, since we've added to the
array object base.

It still needs support in ValueNumber, RangeCheck, CSE, RangeProp.

No diffs.

No asm diffs

Add skeleton for MD array ops morphing.

No diffs

Also,
1. Convert dim/rank types to `unsigned`
2. Assume type of GT_MDARR_LENGTH, GT_MDARR_LOWER_BOUND are TYP_INT
3. Fix: add GT_MDARR_LOWER_BOUND to GenTree::VisitOperands
4. Add dump of GT_MDARR_LENGTH, GT_MDARR_LOWER_BOUND `dim` field to tree dumps (required to distinguish them)
5. Change impArrayAccessIntrinsic() to cast index types to TYP_INT but only under `compJitEarlyExpandMDArrays` to avoid asm diffs.
6. Temporary: create new, unique value numbers for GT_MDARR_LENGTH, GT_MDARR_LOWER_BOUND; otherwise, they get optimized incorrectly by not looking at dimension value.

1. Check for root opts.compJitEarlyExpandMDArrays flag, not in inlinee Compiler
2. Fix order of evaluation for index expressions; copy index expressions with
side effects.

- Change assert that index type is TYP_INT to check genActualType instead of type.
- Change order of evaluation so array is evaluated first, before index expressions.
- Only morph MD array ops if the function and block have them.
- Remove unused `BBF_HAS_NEWARRAY`, add `BBF_HAS_MD_IDX_LEN`, `BBF_HAS_MDARRAYREF`
- Add `OMF_HAS_MDNEWARRAY, `OMF_HAS_MDARRAYREF`
- Add asserts that GT_ARR_ELEM doesn't exist by lowering if we're doing early lowering of MD arrays
- Remove unneeded `ARR_ST_POST_VERIFY`, `ARR_LD_POST_VERIFY`

SimpleLowering is lowering MDARR_LEN, MD_ARR_LOWER_BOUND

New CSEs and hoisted expressions are seen.

Introduce GenTreeArrCommon as parent of GenTreeArrLen, GenTreeMDArr.

Configuration variables:
1. `COMPlus_JitEarlyExpandMDArrays`. Set to zero to disable early MD expansion. Default is 1 (enabled).
2. If `COMPlus_JitEarlyExpandMDArrays=0`, use `COMPlus_JitEarlyExpandMDArraysFilter` to selectively
enable early MD expansion for a method set (e.g., syntax like `JitDump`)