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

[Ying Huang via llvm-commits]

yingopq 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 
```


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