[release/7.0] [mono][ppc64le] Fix wrong implementation of OP_CHECK_THIS #77269
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Backport of #76990 to release/7.0
/cc @lambdageek @Sapana-Khemkar
Customer Impact
Fixes a crash for customers running .NET on the PowerPC architecture in code that calls CoreLib methods that call
SpanHelpers.IndexOfValueType
.In OP_CHECK_THIS access single byte only instead of double word.
This fixes crashes observed in runtime-portable/runtime during source build with rc1 on ppc64le. All these crashes were segmentation faults in SpanHelpers.IndexOfValueType.
Similar issue has been observed on s390 platform as well. Issue was opened https://github.com/dotnet/runtime/issues/76915and fixed by #76916
Testing
Manual testing
Risk
Low. The change is in the PPC-specific part of the mono JIT, and does not affect mobile, WASM or CoreCLR platforms.