[Wasm RyuJIT] Use offset reloc for call indirection-cell loads

[Copilot]

Wasm RyuJIT call codegen dereferences the indirection cell with i32.const <reloc> ; i32.add ; i32.load 0, using a WASM_MEMORY_ADDR_REL_SLEB const reloc. The two scaffolding instructions (i32.const_address, i32.add) can be folded into the load itself by emitting the relocation as the load's offset immediate (WASM_MEMORY_ADDR_REL_LEB), shrinking every helper/indirect call site.

Before:

global.get $__r2r_start
i32.const  <WASM_MEMORY_ADDR_REL_SLEB>
i32.add
i32.load   0
i32.load   0      ; only on EC_INDIR_R
call_indirect

After:

global.get $__r2r_start
i32.load   offset=<WASM_MEMORY_ADDR_REL_LEB>
i32.load   0      ; only on EC_INDIR_R
call_indirect

Changes

• emitwasm.cpp — IF_MEMARG encoding: route the offset immediate through emitOutputConstant(..., UNSIGNED, WASM_MEMORY_ADDR_REL_LEB) so a idIsCnsReloc() instruction emits a padded-ULEB128 reloc in the offset slot; non-reloc cases keep the plain ULEB128 path. Size calculation already accounted for this.
• emitwasm.{h,cpp} — new helper emitter::emitLoadAtAddressConstant(instruction ins, void* address): emits global.get $__r2r_start + <ins> offset=<reloc> for any IF_MEMARG load/store, asserting the format.
• codegenwasm.cpp — genEmitHelperCall: replace both emitAddressConstant(addr) + emitIns_I(INS_I_load, EA_PTRSIZE, 0) pairs (managed-helper PEP push and EC_INDIR_R indirection-cell deref) with emitLoadAtAddressConstant(INS_I_load, addr). The trailing PEP-deref load on EC_INDIR_R is unchanged.

emitAddressConstant is retained for genCodeForConstant, which takes an address rather than dereferencing it.

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