[SPIR-V] Fix generation of invalid SPIR-V in cases of of bitcasts between pointers and multiple null pointers used in the input LLVM IR (#118298) This PR resolved the following issues: (1) There are rare but possible cases when there are bitcasts between pointers intertwined in a sophisticated way with loads, stores, function calls and other instructions that are part of type deduction. In this case we must account for inserted bitcasts between pointers rather than just ignore them. (2) Null pointers have the same constant representation but different types. Type info from Intrinsic::spv_track_constant() refers to the opaque (untyped) pointer, so that each MF/v-reg pair would fall into the same Const record in Duplicate Tracker and would be represented by a single OpConstantNull instruction, unless we use precise pointee type info. We must be able to distinguish one constant (null) pointer from another to avoid generating invalid code with inconsistent types of operands.
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