As part of this effort, remove MipsFCmp nodes and use tablegen
patterns rather than custom lowering through C++.
Unexpectedly, this improves codesize for microMIPS as previous floating
point setcc expansions would materialize 0 and 1 into GPRs before using
the relevant mov[tf].[sd] instruction. Now $zero is used directly.
Nit: I do believe that MMRel is not necessary here. It seems that instruction mapping is not used in this case.