[tablegen] Add !listconcat operator with the similar semantics as !strconcat

Summary:
It concatenates two or more lists. In addition to the !strconcat semantics
the lists must have the same element type.

My overall aim is to make it easy to append to Instruction.Predicates
rather than override it. This can be done by concatenating lists passed as
arguments, or by concatenating lists passed in additional fields.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: hfinkel, llvm-commits

Differential Revision: http://reviews.llvm.org/D3506

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208183 91177308-0d34-0410-b5e6-96231b3b80d8

docs/TableGen/LangIntro.rst

@@ -160,6 +160,12 @@ supported include:
     remaining elements in the list may be arbitrary other values, including
     nested ```dag``' values.
 
+``!listconcat(a, b, ...)``
+    A list value that is the result of concatenating the 'a' and 'b' lists.
+    The lists must have the same element type.
+    More than two arguments are accepted with the result being the concatenation
+    of all the lists given.
+
 ``!strconcat(a, b, ...)``
     A string value that is the result of concatenating the 'a' and 'b' strings.
     More than two arguments are accepted with the result being the concatenation

docs/TableGen/LangRef.rst

@@ -93,7 +93,7 @@ wide variety of meanings:
    BangOperator: one of
                :!eq     !if      !head    !tail      !con
                :!add    !shl     !sra     !srl
-               :!cast   !empty   !subst   !foreach   !strconcat
+               :!cast   !empty   !subst   !foreach   !listconcat   !strconcat
 
 Syntax
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