Incorrect llvm.minimum/llvm.maximum to fmin/fmax ExtInst mapping wrt NaN handling

[svenvh]

In SPIRVWriter.cpp, it's incorrect to map minimum to OpenCLLIB::Fmin due to differences in NaN behaviour:

llvm.minimum:

If either operand is a NaN, returns NaN.

fmin:

If one operand is a NaN, fmin returns the other argument. If both arguments are NaNs, fmin returns a NaN.

[wenju-he]

it is also incorrect to map minnum to OpenCLLIB::Fmin at

SPIRV-LLVM-Translator/lib/SPIRV/SPIRVWriter.cpp

Lines 4073 to 4074 in 43a5855

	case Intrinsic::minnum:
	return OpenCLLIB::Fmin;