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

[Jay Foad via llvm-commits]

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@@ -114,6 +114,11 @@ HANDLE_TARGET_OPCODE(REG_SEQUENCE)
 /// used to copy between subregisters of virtual registers.
 HANDLE_TARGET_OPCODE(COPY)
 
+/// COPY_LANEMASK - Target-independent register copy for active mask in 
+/// register as represented by the lanemask. This instruction does not 
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jayfoad wrote:

Suggestion:

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.

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