[Mosaic GPU] Add an initial skeleton for an equation system for layout/transform inference.
The idea will be to build an `EquationSystem` that describes how the layouts
of the operands/results of MLIR operations relate to each other. Eventually,
we'll be able to use an equation system to connect layouts in different
memory spaces---e.g. `transforms` in SMEM, layouts in registers, and even
TMEM layouts.
PiperOrigin-RevId: 777562855