Enhance Intrinsic::fmuladd translation.

-cl-mad-enable is off by default, however ReplaceLLVMFmulAddWithOpenCLMad is true by default https://github.com/KhronosGroup/SPIRV-LLVM-Translator/blob/main/include/LLVMSPIRVOpts.h#L244C8-L244C40, which seems not consistent. Intrinsic::fmuladd should be translated into mad when function attribute "less-precise-fpmad" = "true" which indicates -cl-mad-enable is specified. When "less-precise-fpmad" = "false" or there is no "less-precise-fpmad", translate it into a pair of fmul + fadd.
Can we remove ReplaceLLVMFmulAddWithOpenCLMad uses and let only "less-precise-fpmad" determine how to translate fmuladd?
@AlexeySachkov, @MrSidims, @svenvh , do you think is ReplaceLLVMFmulAddWithOpenCLMad still needed and what's your opinion for above comments? Thanks very much. https://github.com/KhronosGroup/SPIRV-LLVM-Translator/pull/2163 is drafted and need further discussion.

Start with include/LLVMSPIRVOpts.h and the uses of ReplaceLLVMFmulAddWithOpenCLMad, then trace the Intrinsic::fmuladd translation path. Review the drafted pull request #2163 and the existing discussion before deciding whether the less-precise-fpmad attribute alone should control translation. Done means the behavior is agreed, implemented, and covered by the relevant translator tests.

Written by the indexing model from the issue text.