[llvm] [RISCV] Add a pass to eliminate special copies in order to facilitate shrink-wrap optimization (PR #140716)

[Sam Elliott via llvm-commits]

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@@ -0,0 +1,151 @@
+//===- RISCVCopyCombine.cpp - Remove special copy for RISC-V --------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass attempts a shrink-wrap optimization for special cases, which is
+// effective when data types require extension.
+//
+// After finalize-isel:
+//   bb0:
+//   liveins: $x10, $x11
+//     %1:gpr = COPY $x11   ---- will be delete in this pass
+//     %0:gpr = COPY $x10
+//     %2:gpr = COPY %1:gpr ---- without this pass, sink to bb1 in machine-sink,
+//                               then delete at regalloc
+//     BEQ %0:gpr, killed %3:gpr, %bb.3 PseudoBR %bb1
----------------
lenary wrote:

```suggestion
//     BEQ %0:gpr, killed %3:gpr, %bb.3
//     PseudoBR %bb1
```

https://github.com/llvm/llvm-project/pull/140716