[clang] [llvm] [AArch64][SME] Remove SelectionDAG SME ABI lowering (PR #190950)

[Benjamin Maxwell via cfe-commits]

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@@ -50,8 +50,6 @@ SMEAttrs::SMEAttrs(const AttributeList &Attrs) {
     Bitmask |= SM_Body;
   if (Attrs.hasFnAttr("aarch64_za_state_agnostic"))
     Bitmask |= ZA_State_Agnostic;
-  if (Attrs.hasFnAttr("aarch64_zt0_undef"))
-    Bitmask |= ZT0_Undef;
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MacDue wrote:

Okay, I've added an entry to `AArch64SME.rst` and kept a dummy check in `SMEAttrs::SMEAttrs`. 

https://github.com/llvm/llvm-project/pull/190950