APFloat incorrectly generates signaling NaNs on MIPS

[Muon]

The signaling bit convention on MIPS is opposite that of x86 and ARM, but APFloat doesn't know that. As a result, constant folding to a NaN erroneously produces a signaling NaN instead of a quiet NaN on MIPS: https://godbolt.org/z/TP5hePhWP

[llvmbot]

@llvm/issue-subscribers-backend-mips

[RalfJung]

The signaling bit convention on MIPS

From what I understand, this is true for some MIPS chips, but other more recent chips actually make this runtime-configurable. For those chips it is impossible to know at compiletime which NaN needs to be produced to get a quiet NaN, isn't it?

[wzssyqa]

For MIPSr2/r3/r5, we have an option -mnan=2008/legacy to swtich it.
For r6, 2008 is required.