nir: add rcp(w) lowering for gl_FragCoord

nir: add rcp(w) lowering for gl_FragCoord

On some hardware (e.g. Mali400) the shader needs to apply some
transformations for correct gl_FragCoord handling. The lowering
actions look like the following in pseudocode:
   gl_FragCoord.xyz = gl_FragCoord_orig.xyz
   gl_FragCoord.w = 1.0 / gl_FragCoord_orig.w

Add this lowering as a nir pass in preparation for using it in the driver.

Signed-off-by: Andreas Baierl <ichgeh@imkreisrum.de>
Reviewed-by: Qiang Yu <yuq825@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>