[InstCombine] Make use of low zero bits to determine exact int->fp cast

According the comment https://reviews.llvm.org/D127854#inline-1226805,
We could also make use of these low zero bits, https://alive2.llvm.org/ce/z/GYxTRu

Reviewed By: spatel, nikic, xbolva00

Differential Revision: https://reviews.llvm.org/D128895