vc4: Avoid tlb_c_ms for same-value writes in MSAA shaders

[anholt]

With MSAA on and blending depending on the destination color, then we emit the blend code per sample instead of per fragment. However, our test in In vc4_nir_lower_blend.c:blend_depends_on_dst_color() is too conservative, and so we emit per-sample tlb_c_ms in many cases where the color value is the same on all samples. This particularly impacts webgl shaders, where trivial blending is often enabled and MSAA is usually enabled.