[llvm] [X86][MC] Support Intel FRED and LKGS instructions. (PR #91909)

[Shengchen Kan via llvm-commits]

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@@ -68,6 +68,12 @@ def SYSENTER : I<0x34, RawFrm, (outs), (ins), "sysenter", []>, TB;
 def SYSEXIT   : I<0x35, RawFrm, (outs), (ins), "sysexit{l}", []>, TB;
 def SYSEXIT64 :RI<0x35, RawFrm, (outs), (ins), "sysexitq", []>, TB,
                   Requires<[In64BitMode]>;
+
+// FRED Instructions
+def ERETS: I<0x01, MRM_CA, (outs), (ins), "erets",
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KanRobert wrote:

> The following sections indicate situations in which one of the instructions may cause a
> control-protection exception (#CP). 

We should set hasSideEffects = 1 for them

https://github.com/llvm/llvm-project/pull/91909