intel/fs: Do not lower large local arrays to scratch on gen7

On gen7 and earlier the scratch space size is limited to 12kB. By enabling this optimization we may easily exceed this limit without having any fallback.

arb_compute_shader/linker/bug-93840.shader_test crashes with this lowering on IVB due to exceeding scratch size limit.

Closes: #2092 (closed) Fixes: 69244fc7 Signed-off-by: Danylo Piliaiev danylo.piliaiev@globallogic.com