[llvm] [Mips] Add lowerConstantFP to optimize some floating load instrs to mtc1+mthc1 (PR #196226)

[Jiaxun Yang via llvm-commits]

FlyGoat wrote:

> > Would also work?
> 
> It will generate the following asm, and will involve one additional instruction `add`.
> 
> ```
> addiu $3, $zero, 0 
> mtc1 $3, $f0 
> mthc1 $2, $f0 
> ```

Then maybe:

```
SDValue Lo = DAG.getCopyFromReg(DAG.getEntryNode(), DL, Mips::ZERO, MVT::i32);
```

:-)

A side note is `DAG.getConstant(0)` optimisation need to be fixed at some point? Maybe add a TODO here.

Thanks

https://github.com/llvm/llvm-project/pull/196226