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

[Jay Foad via llvm-commits]

================
@@ -807,6 +807,21 @@ For an int eq predicate ``ICMP_EQ``, the syntax is:
 
    %2:gpr(s32) = G_ICMP intpred(eq), %0, %1
 
+Lanemask Operands
+^^^^^^^^^^^^^^^^^^
+
+A Lanemask operand is 64-bit unsigned value that can the store lane information 
+for a register operand in the instruction. It can be used as many times as needed
+in an instruction, with one (atleast) or more register operands associated with it.
----------------
jayfoad wrote:

"At least" is two words, but here it is redundant anyway:
```suggestion
in an instruction, with one or more register operands associated with it.
```

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