[globalisel][tablegen] Ensure MatchTable's are compile-time constants with constexpr. NFC.
[globalisel][tablegen] Ensure MatchTable's are compile-time constants with constexpr. NFC.

This should prevent any re-occurence of the problem where the table was
initialized at run-time.



git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@309267 91177308-0d34-0410-b5e6-96231b3b80d8

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