[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/apu/xma_context.cc

@@ -921,7 +921,7 @@ void XmaContext::ConvertFrame(const uint8_t** samples, bool is_two_channel,
       auto in = reinterpret_cast<const float*>(samples[j]);
 
       // Raw samples sometimes aren't within [-1, 1]
-      float scaled_sample = xe::saturate_signed(in[i]) * scale;
+      float scaled_sample = xe::clamp_float(in[i], -1.0f, 1.0f) * scale;
 
       // Convert the sample and output it in big endian.
       auto sample = static_cast<int16_t>(scaled_sample);