Fixes#138997 and does refactoring for low-precision-related flags.
The shader feature flags MinimumPrecision and NativeLowPrecision were
not being set, leading to validation errors.
This PR sets these shader feature flags [as in
DXC](377c4ca6d8/lib/DXIL/DxilShaderFlags.cpp (L58-L63))
and adds tests for them.
This PR also performs some refactoring of low-precision-related flags to
make it less confusing.
- The `UseNativeLowPrecision` DXIL module flag has been renamed to
`NativeLowPrecisionMode` to imply that it is setting some execution
state which the module should be interpreted with
- The LLVM module flag `dx.nativelowprec` is now read only once and sets
a bool to be used by `updateFunctionFlags()` and for setting the DXIL
module flag `NativeLowPrecisionMode`
- The `MinimumPrecision`, `NativeLowPrecision`, and
`LowPrecisionPresent` shader feature flags are all set together under
`updateFunctionFlags()`
- Moved the logic for setting DXIL module flags `NativeLowPrecisionMode`
and `ResMayNotAlias` out of the per-function loop and placed it
alongside the logic for setting other DXIL module flags
(`DisableOptimizations`, `Max64UAVs`, and `UAVsAtEveryStage` flags)
---------
Co-authored-by: Justin Bogner <mail@justinbogner.com>
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