microsoft/compiler: Duplicate some SSA values to simplify SSA typing

microsoft/compiler: Duplicate some SSA values to simplify SSA typing

For each phi src, ensure that it's only used as a phi src.
This lets us give each phi their own unique types without worrying
about them stomping on each other. Also scalarize phis.

For each constant, ensure that it's only used once. The DXIL backend
will already dedupe these consts within the module, but this lets a
single load_const have multiple types depending on how it's used.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22972>