[Base/GPU] Cleanup float comparisons and NaN and -0 in clamping

C++ relational operators are supposed to raise FE_INVALID if an argument is
NaN, use std::isless/greater[equal] instead where they were easy to locate
(though there are other places possibly, mostly min/max and clamp usage was
checked).

Also fixes a copy-paste error making the CPU shader interpreter execute
MINs as MAXs instead.

src/xenia/gpu/xenos.cc

@@ -27,7 +27,7 @@ namespace xenos {
 float PWLGammaToLinear(float gamma) {
   // Not found in game executables, so just using the logic similar to that in
   // the Source Engine.
-  gamma = xe::saturate_unsigned(gamma);
+  gamma = xe::saturate(gamma);
   float scale, offset;
   // While the compiled code for linear to gamma conversion uses `vcmpgtfp
   // constant, value` comparison (constant > value, or value < constant), it's
@@ -68,7 +68,7 @@ float PWLGammaToLinear(float gamma) {
 }
 
 float LinearToPWLGamma(float linear) {
-  linear = xe::saturate_unsigned(linear);
+  linear = xe::saturate(linear);
   float scale, offset;
   // While the compiled code uses `vcmpgtfp constant, value` comparison
   // (constant > value, or value < constant), it's preferable to use `value >=