[InstCombine] Add folds for `(fp_binop ({s|u}itofp x), ({s|u}itofp y))`

The full fold is one of the following:
1) `(fp_binop ({s|u}itofp x), ({s|u}itofp y))`
    -> `({s|u}itofp (int_binop x, y))`
2) `(fp_binop ({s|u}itofp x), FpC)`
    -> `({s|u}itofp (int_binop x, (fpto{s|u}i FpC)))`

And support the following binops:
    `fmul` -> `mul`
    `fadd` -> `add`
    `fsub` -> `sub`

Proofs: https://alive2.llvm.org/ce/z/zuacA8

The proofs timeout, so they must be reproduced locally.

Closes #82555