volumerhs performance regression

[maleadt]

GPUCompiler v0.20.2 ⇒ v0.20.3 regressed volumerhs from 86 registers to 110 registers.

[maleadt]

Reduced to JuliaGPU/GPUCompiler.jl@4f4b991, so this was only a temporary improvement (in a bad way). #1895 may be a better solution instead.