Add support for expanding fptosi.sat and fptoui.sat via IR expansions.
Similar to fptosi/fptoui we would get legalization errors otherwise.
The previous expansion for fptosi/fptoui was already saturating -- but
those instructions do not actually require saturation, and the
implementation of the saturation was incorrect in lots of ways. What
this PR does is:
* For fptosi, remove the unnecessary saturation handling.
* For fptoui, remove the unnecessary saturation handling and sign
multiplication.
* For fptosi, use the previous saturation handling with fixes: We need
to map NaNs to 0 and the saturation condition on the exponent was
incorrect. (I'm performing the NaN check via fcmp -- there's no
requirement to do everything bitwise here.)
* For fptoui use a variation of the signed saturation handling: Negative
values need to go to zero and we saturate to unsigned max.
Proofs: https://alive2.llvm.org/ce/z/Xv9FNd
a8f211904a