[mlir] Make 1:N type conversion tests less brittle.

They had previously not followed the best practices; I must have
commited these in a rush. In particular, they used hard-coded SSA names
instead of patterns. This patch changes that, uses CHECK-DAG more
instead of CHECK-NEXT, and makes all function names unique.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D148012