[LangRef] Clarify that selects can have !unpredictable (#162753)

Originally `!unpredictable` could only appear on branches and switches,
but now it can also appear on selects. This change updates the LangRef
accordingly.

Author: tcreech-intel

llvm/docs/LangRef.rst

@@ -7517,12 +7517,12 @@ sections that the user does not want removed after linking.
 '``unpredictable``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-``unpredictable`` metadata may be attached to any branch or switch
-instruction. It can be used to express the unpredictability of control
-flow. Similar to the ``llvm.expect`` intrinsic, it may be used to alter
-optimizations related to compare and branch instructions. The metadata
-is treated as a boolean value; if it exists, it signals that the branch
-or switch that it is attached to is completely unpredictable.
+``unpredictable`` metadata may be attached to any branch, select, or switch
+instruction. It can be used to express the unpredictability of control flow.
+Similar to the ``llvm.expect`` intrinsic, it may be used to alter optimizations
+related to compare and branch instructions. The metadata is treated as a
+boolean value; if it exists, it signals that the branch, select, or switch that
+it is attached to is completely unpredictable.
 
 .. _md_dereferenceable: