[llvm] [RISCV] Use BuildPairF64 and SplitF64 for bitcast i64<->f64 on rv32 regardless of Zfa. (PR #85982)

[Craig Topper via llvm-commits]

topperc wrote:

This trips the subregister assertions in `getRegAllocationHints` for this test case with Zdinx.

```
define double @foo(i64 noundef %0) {     
  %2 = add i64 %0, 1                                                             
  %3 = bitcast i64 %2 to double                                                  
  %4 = fadd double %3, 1.000000e+00                                              
  ret double %4                                                                  
}
```

https://github.com/llvm/llvm-project/pull/85982