radv,aco,nir: lower sample adjustment in NIR, remove for storage image loads
Lowering the sample index adjustment in NIR simplifies ACO and will let us optimize the FMASK fetch (for example, move it out of loops). It will also simplify future descriptor lowering in NIR.
The sample index adjustment we performed for mutlisampled image loads seems unnecessary, since we always expand the FMASK when transitioning to a layout which supports storage images.