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

[Jay Foad via llvm-commits]

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@@ -807,6 +807,23 @@ For an int eq predicate ``ICMP_EQ``, the syntax is:
 
    %2:gpr(s32) = G_ICMP intpred(eq), %0, %1
 
+LaneMask Operands
+^^^^^^^^^^^^^^^^^^
+
+A LaneMask operand contains a LaneBitmask struct representing the covering of a
+register with sub-registers. [That's what it says in LaneBitmask.h!] Instructions
+typically associate a LaneMask operand with one or more Register operands, and
+use it to represent sub-register granularity information like liveness for those 
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jayfoad wrote:

```suggestion
use it to represent sub-register granularity information like liveness for those
```

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