Lower affine control flow to std control flow to LLVM dialect

This CL splits the lowering of affine to LLVM into 2 parts:
1. affine -> std
2. std -> LLVM

The conversions mostly consists of splitting concerns between the affine and non-affine worlds from existing conversions.
Short-circuiting of affine `if` conditions was never tested or exercised and is removed in the process, it can be reintroduced later if needed.

LoopParametricTiling.cpp is updated to reflect the newly added ForOp::build.

PiperOrigin-RevId: 257794436