[DebugInfo][InstrRef] Handle llvm.frameaddress intrinsics gracefully
When working out which instruction defines a value, the instruction-referencing variable location code has a few special cases for physical registers: * Arguments are never defined by instructions, * Constant physical registers always read the same value, are never def'd This patch adds a third case for the llvm.frameaddress intrinsics: you can read the framepointer in any block if you so choose, and use it as a variable location, as shown in the added test. This rather violates one of the assumptions behind instruction referencing, that LLVM-ir shouldn't be able to read from an arbitrary register at some arbitrary point in the program. The solution for now is to just emit a DBG_PHI that reads the register value: this works, but if we wanted to do something clever with DBG_PHIs in the future then this would probably get in the way. As it stands, this patch avoids a crash. Differential Revision: https://reviews.llvm.org/D106659
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