[AMDGPU] expand-fp: always report modifications (#163153)
The last change to the pass in PR #158588 lost the assignment to the
"Modified" variable for one of the pass optimizations.
Add it back. This fixes the test failure in
`CodeGen/AMDGPU/itofp.i128.bf.ll` (in a
`LLVM_ENABLE_EXPENSIVE_CHECKS=ON` build).
