nir/i965: assert first is always less than 64
nir/i965: assert first is always less than 64

This fixes a defect detected by Coverity Scan.

Reviewed-by: Iago Toral Quiroga <a847aabaac44184372f6b1cd1a59aa254daaeb0b@igalia.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler