Merge LowerAffineApplyPass into LowerIfAndForPass, rename to LowerAffinePass

This change is mechanical and merges the LowerAffineApplyPass and
LowerIfAndForPass into a single LowerAffinePass.  It makes a step towards
defining an "affine dialect" that would contain all polyhedral-related
constructs.  The motivation for merging these two passes is based on retiring
MLFunctions and, eventually, transforming If and For statements into regular
operations.  After that happens, LowerAffinePass becomes yet another
legalization.

PiperOrigin-RevId: 227566113