[llvm] [CodeGen] Add MO_LaneMask type and a new COPY_LANEMASK instruction (PR #151944)

[Vikash Gupta via llvm-commits]

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@@ -114,6 +114,12 @@ HANDLE_TARGET_OPCODE(REG_SEQUENCE)
 /// used to copy between subregisters of virtual registers.
 HANDLE_TARGET_OPCODE(COPY)
 
+/// COPY_LANEMASK - Target-independent partial register copy. The laneMask
+/// operand indicates which parts of the source register are copied to the
+/// destination. Other parts of the destination are undefined. It does not
+/// support copy between subregisters of virtual registers.
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vg0204 wrote:

What I really meant to say is that those virtual registers which have subreg indices like %0:vgpr64.sub0, as in this case LaneMask's value would have ambiguity in interpretation.

https://github.com/llvm/llvm-project/pull/151944