[mlir] Support return and call ops in bare-ptr calling convention

This patch adds support for the 'return' and 'call' ops to the bare-ptr
calling convention. These changes also align the bare-ptr calling
convention code with the latest changes in the default calling convention
and reduce the amount of customization code needed.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D87724