[SDAG] fix miscompile when casting int->FP->int

This is the codegen equivalent of D124692.

As shown in https://github.com/llvm/llvm-project/issues/55150 -
the existing fold may be wrong when converting to a signed value.
This is a quick fix to avoid the miscompile.
https://alive2.llvm.org/ce/z/KtaDmd

Differential Revision: https://reviews.llvm.org/D124771