[SelectionDAG] Disallow indirect "i" constraint

This allows us to delete InlineAsm::Constraint_i workarounds in
SelectionDAGISel::SelectInlineAsmMemoryOperand overrides and
TargetLowering::getInlineAsmMemConstraint overrides.

They were introduced to X86 in r237517 to prevent crashes for
constraints like "=*imr". They were later copied to other targets.