IRInterpreter now recognizes expressions with constants it doesn't handle.

If an instruction has a constant that IRInterpreter doesn't know how to deal
with (say, an array constant, because we can't materialize it to APInt) then we
used to ignore that and only fail during expression execution.  This is annoying
because if IRInterpreter had just returned false from CanInterpret(), the JIT
would have been used.

Now the IRInterpreter checks constants as part of CanInterpret(), so this should
hopefully no longer be an issue.

llvm-svn: 260735