Replace regNum with GetRegNum() (dotnet/coreclr#27120)

Replace regNum with GetRegNum() (dotnet/coreclr#27120)

* find src/jit -type f -exec sed -i -e 's/->regNum/->getRegNum()/g' {} \;

Add forgotten regNum<-> getRegNum

* GetRegNum fix

* more forgotten

* more forgotten

Commit migrated from https://github.com/dotnet/coreclr/commit/3219e8b5633522c9ef39e39ee99974c71f5e928f