Remove immarg from llvm.expect

The LangRef claimed this was required to be a constant, but this
appears to be wrong.

Fixes bug 41079.

llvm-svn: 356353
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index 009f855..fcea8a4 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -15819,8 +15819,7 @@
 """"""""""
 
 The ``llvm.expect`` intrinsic takes two arguments. The first argument is
-a value. The second argument is an expected value, this needs to be a
-constant value, variables are not allowed.
+a value. The second argument is an expected value.
 
 Semantics:
 """"""""""