fix several compile issues with Visual Studio toolchains

[bashbaug]

Fixes several compile issues I am seeing for my version of Visual Studio related to an ambiguous call to fpclassify, which is called by isnan and other similar functions, specifically for the cl_half type:

19>C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\corecrt_math.h(401,1): error C2668: 'fpclassify': ambiguous call to overloaded function
19>C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\corecrt_math.h(298,31): message : could be 'int fpclassify(long double) throw()'
19>C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\corecrt_math.h(293,31): message : or       'int fpclassify(double) throw()'
19>C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\corecrt_math.h(288,31): message : or       'int fpclassify(float) throw()'
19>C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\corecrt_math.h(401,1): message : while trying to match the argument list '(_Ty)'

Some of these issues seem like differences in compiler behavior, but at least one appears to have identified a legitimate bug.

Specifically, this change:

• Removes the special-case checks for finite half numbers for commonfns, since this is already handled by UlpFn. (test with: test_commonfns degrees radians)
• Assigns to temporary variables to eliminate the ambiguous function call for relationals. (test with: test_relationals relational*)
• Properly converts from half to float when checking for NaNs for select. This is the one that seems like a legitimate bug. (test with: test_select select_half_ushort select_half_short)
• Uses std::enable_if to disambiguate a function call for spirv_new. (test with: test_spirv_new decorate_saturated*)

If it's helpful, my specific Visual Studio version is:

Microsoft Visual Studio Professional 2019
Version 16.11.20
VisualStudio.16.Release/16.11.20+32929.386

I also have the Windows Software Development Kit 10.0.19041.685 installed.