[llvm] [GlobalISel] Add G_UITOFP, G_SITOFP to computeKnownBits (PR #215766)

[Eric Ross via llvm-commits]

ZERICO2005 wrote:

> I think negative infinity is also possible for a sufficiently degenerate type combination

This would be relevant for `ieee_half`, where the largest value is `65504`. 
If we do test that, it would be good to test these inputs:
- `(half)65505` rounds to infinity is rounding away from zero.
- `(half)65519` rounds to `HALF_MAX` under ties-to-even.
- `(half)65520` rounds to infinity under ties-to-even.
- `(half)65535` rounds to `HALF_MAX` if rounding towards zero.


https://github.com/llvm/llvm-project/pull/215766