gpu: Use `@interpolate(flat, either)` for non-varying values. #853

Job	Run time
incremental-mutants	0s
deny	47s
lint (stable)	2m 59s
build (ubuntu, stable, locked, true)	8m 19s
lint (nightly)	5m 44s
build (windows, stable, locked, false)	16m 50s
lint (beta)	7m 36s
miri	1m 52s
build (macos, stable, locked, false)	7m 27s
no_std	55s
build (ubuntu, nightly, locked, false)	18m 54s
build (ubuntu, beta, locked, false)	22m 44s
build (ubuntu, stable, latest, false)	13m 33s
build (ubuntu, stable, minimal, false)	13m 8s
features	24m 52s
fuzz	15m 23s
deploy	8s
	2h 41m 11s

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