Generate (old + 0) instead of Sp in stack checks
When compiling a function we can determine how much stack space it will use. We therefore need to perform only a single stack check at the beginning of a function to see if we have enough stack space. Instead of referring directly to Sp - as we used to do in the past - the code generator uses (old + 0) in the stack check. Stack layout phase turns (old + 0) into Sp. The idea here is that, while we need to perform only one stack check for each function, we could in theory place more stack checks later in the function. They would be redundant, but not incorrect (in a sense that they should not change program behaviour). We need to make sure however that a stack check inserted after incrementing the stack pointer checks for a respectively smaller stack space. This would not be the case if the code generator produced direct references to Sp. By referencing (old + 0) we make sure that we always check for a correct amount of stack: when converting (old + 0) to Sp the stack layout phase takes into account changes already made to stack pointer. The idea for this change came from observations made while debugging #8275.
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