[llvm] [SPIR-V] Add pass to remove spv_ptrcast intrinsics (PR #128896)

[Steven Perron via llvm-commits]

================
@@ -0,0 +1,265 @@
+//===-- SPIRVLegalizePointerLoad.cpp ----------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// The LLVM IR has multiple legal patterns we cannot lower to Logical SPIR-V.
+// This pass modifies such loads to have an IR we can directly lower to valid
+// logical SPIR-V.
+// OpenCL can avoid this because they rely on ptrcast, which is not supported
+// by logical SPIR-V.
+//
+// This pass relies on the assign_ptr_type intrinsic to deduce the type of the
+// pointed values, must replace all occurences of `ptrcast`. This is why
+// unhandled cases are reported as unreachable: we MUST cover all cases.
----------------
s-perron wrote:

```suggestion
// This pass relies on the assign_ptr_type intrinsic to deduce the type of the
// pointee. All occurrences of `ptrcast` must be replaced because the lead to
// invalid SPIR-V. Unhandled cases result in an error.
```

https://github.com/llvm/llvm-project/pull/128896