missed instcombine fptoui(fadd(uitofp(x), c)) ->x if 0<= c<1

```llvm
define i16 @uitofp_fadd_half_fptoui(i16 %x) {
%f = uitofp i16 %x to float
%rounded = fadd float %f, 5.000000e-01
%back = fptoui float %rounded to i16
ret i16 %back
}
```

can be transformed to

```llvm
define i16 @uitofp_fadd_half_fptoui(i16 %x) {
ret i16 %x
}
```

Start by reproducing the provided LLVM IR example and inspect the InstCombine handling for the uitofp, fadd, and fptoui sequence. Done means the example is reliably transformed to return %x, with coverage for the stated 0 <= c < 1 case.

Written by the indexing model from the issue text.