The NIR and TGSI paths are currently intertwined which makes it
not only hard to follow but also makes it hard to take advantage
of the differences in IR.

Here we take the first step to splitting that path apart. With
this we take the opportunity to no longer call the GLSL IR
optimisation passes after the final lowering calls for NIR. We
can instead just use the NIR passes which can produce better code
and should also result in faster compile times.

The speed-up can be measured in some dolphin uber shaders due to
no longer calling lower_if_to_cond_assign() for example
dolphin/ubershaders/120.shader_test goes from ~1.63 -> ~1.53
seconds on my machine.

There are some code changes as a result of not calling
lower_if_to_cond_assign(), this is because it flattens ifs that
contain UBOs where as NIR's peephole select doesn't. This is
were most of the regressions in Max Waves happens with shader-db.

shader-db results (VEGA):

Totals from affected shaders:
SGPRS: 2349056 -> 2349640 (0.02 %)
VGPRS: 1322160 -> 1323300 (0.09 %)
Spilled SGPRs: 21190 -> 21527 (1.59 %)
Spilled VGPRs: 99 -> 99 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 72 -> 72 (0.00 %) dwords per thread
Code Size: 57260904 -> 57270932 (0.02 %) bytes
Compile Time: 1107186 -> 1022942 (-7.61 %) milliseconds
LDS: 786 -> 786 (0.00 %) blocks
Max Waves: 391932 -> 391619 (-0.08 %)
Wait states: 0 -> 0 (0.00 %)

Reviewed-by: Eric Anholt <eric@anholt.net>