lima/ppir: add NIR pass to split varying loads

lima/ppir: add NIR pass to split varying loads

NIR may emit a single instrinsic to load several packed varyings,
but that's suboptimal for Utgard PP for several reasons:
- varyings that are used as sampler inputs can be passed using
  pipeline register with increased precision
- we have small number of regs, so using a vec4 regs for storing
  two vec2 varyings increases reg pressure.

Add NIR pass to split a single load into several loads and utilize
it in lima.

Reviewed-by: Qiang Yu <yuq825@gmail.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>