[mlir] ConvertStandardToLLVM: do not rely on command line options internally

The patterns for converting `std.alloc` and `std.dealoc` can be configured to
use `llvm.alloca` instead of calling `malloc` and `free`. This configuration
has been only possible through a command-line flag, despite the presence of a
(misleading) parameter in the pass constructor. Use the parameter instead and
only initalize it from the command line flags if the pass is constructed from
the mlir-opt registration.