[IndVars] Eliminate redundant type cast between integer and float

Recompute the range: match for fptosi of sitofp, and then query the range of the input to the sitofp
according the comment on D129140.

Fixes https://github.com/llvm/llvm-project/issues/55505.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D129191