[LoongArch] Fix codegen for [su]itofp instructions

This patch fixes codegen for `[su]itofp` instructions.

In LoongArch, a legal int-to-float conversion is done in two steps:

1. Move the data from `GPR` to `FPR`. (FRLen >= GRLen)
2. Conversion in `FPR`. (the data in `FPR` is treated as a signed value)

Based on the above features, when the type's BitWidth meets the
requirements, all `SINT_TO_FP` are legal, all `UINT_TO_FP` are expand
and lowered to libcall when appropriate.
The only special case is, LoongArch64 with `+f,-d` features. At this
point, custom processing is required for `[SU]INT_TO_FP`. Of course, we
can also ignore it and use libcall directly.

Differential Revision: https://reviews.llvm.org/D136916