[mlir] support reductions in loop to std conversion

Summary:
Introduce support for converting loop.for operations with loop-carried values
to a CFG in the standard dialect. This is achieved by passing loop-carried
values as block arguments to the loop condition block. This block dominates
both the loop body and the block immediately following the loop, so the
arguments of this block are remain visible there.

Differential Revision: https://reviews.llvm.org/D75513