JIT: fold wasm R2R address constants into load memarg offsets

[AndyAyersMS]

Contain a relocatable address constant under a load and emit the relocation in the memarg offset instead of a separate i32.const/i32.add. Saves 814,971 bytes (3.60%) of the System.Private.CoreLib wasm code section.

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[Copilot]

Pull request overview

This PR updates the WASM JIT lowering + codegen/emitter pipeline so that certain relocatable address constants (icon handles) can be contained under GT_IND loads and then emitted as a relocation in the wasm load’s memarg offset, avoiding the separate i32.const <reloc>; i32.add sequence.

Changes:

• Teach Lowering::ContainCheckIndir (WASM) to contain eligible relocatable icon-handle address constants for GT_IND loads (excluding SIMD12 and multiply-used cases).
• Add emitter helpers to (a) emit the module base (imageBase) and (b) emit a memarg whose offset is a relocatable address constant.
• Update genCodeForIndir to recognize the contained icon-handle address case and emit imageBase + load(memarg offset=<reloc>).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/lowerwasm.cpp	Adds containment for relocatable icon-handle addresses under eligible GT_IND loads so codegen can fold them into memarg offsets.
src/coreclr/jit/emitwasm.h	Declares new emitter helpers (emitImageBase, emitIns_MemargAddress) used by WASM codegen.
src/coreclr/jit/emitwasm.cpp	Implements emitImageBase / emitIns_MemargAddress and switches IF_MEMARG emission to use relocation-aware constant output when needed.
src/coreclr/jit/codegenwasm.cpp	Uses the new contained-address pattern in genCodeForIndir to emit imageBase plus a memarg relocation for loads.

[AndyAyersMS]

@adamperlin PTAL
fyi @dotnet/wasm-contrib

Handles folding offsets into global loads (where we know overflow is not possible).

Size

System.Private.CoreLib, crossgen2 -O, --targetos:browser --targetarch:wasm:

	code section
baseline	22,635,652
this change	21,820,681
	-814,971 (-3.60 %)

271,657 sites folded, each saves 3 bytes.

Example

System.Text.DecoderExceptionFallback.CreateFallbackBuffer goes from 83 -> 83 bytes

;; before                                    ;; after
0026  local.get 0                            0026  local.get 0
0028  global.get 1      ;; $imageBase        0028  global.get 1                    ;; $imageBase
002e  i32.const 7385676 ;; reloc             002e  i32.load align=0 offset=7385676 ;; reloc
0034  i32.add
0035  i32.load align=0 offset=0
0038  global.get 1                           0035  global.get 1
003e  i32.const 7292488 ;; reloc             003b  i32.load align=0 offset=7292488 ;; reloc
0044  i32.add
0045  i32.load align=0 offset=0
0048  local.set 3                            0042  local.set 3