[mlir] Turn flags in ConvertStandardToLLVM into pass flags

Follow-up on D72802. Turn -convert-std-to-llvm-use-alloca and
-convert-std-to-llvm-bare-ptr-memref-call-conv into pass flags
of LLVMLoweringPass.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D73912