This commit updates the LLVM dialect CallOp and InvokeOp to always print
the variadic callee type (previously callee type) if present. An
additional verifier checks that only variadic calls have a non-null
variadic callee type, and the builders are adapted accordingly to set
the variadic callee type for variadic calls only. Finally, the CallOp
and InvokeOp verifiers are strengthened to check that the variadic
callee type matches the call argument and result types.
The motivation of this change is that CallOp and InvokeOp don't have
hidden state that is not pretty printed, but used during the export to
LLVM IR. Previously, it could happen that a call looked correct in MLIR,
but the return type changed after exporting to LLVM IR (since it has
been taken from the hidden callee type attribute). After landing this
change, this is not possible anymore since the variadic callee type is
always printed if present.
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