[maglev] Avoid duplicated sign extension in LoadTaggedFieldByFieldIndex

Change-Id: Id9ede5a54edcdbadf4ec3a4989a37158bd954d88
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/5625369
Commit-Queue: Hao A Xu <hao.a.xu@intel.com>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#94391}

src/maglev/maglev-ir.cc

@@ -2425,7 +2425,6 @@ void LoadTaggedFieldByFieldIndex::GenerateCode(MaglevAssembler* masm,
         // We haven't untagged, so we need to sign extend.
         __ SignExtend32To64Bits(field_index, field_index);
       }
-      __ SignExtend32To64Bits(field_index, field_index);
       __ LoadTaggedFieldByIndex(
           result_reg, object, field_index, scale,
           JSObject::kHeaderSize - (kIsDoubleBit << (kTaggedSizeLog2 - 1)));