Revert "[SCCP] convert signed div/rem to unsigned for non-negative operands"

This reverts commit fe1f3cfc.

It looks like this commit breaks building llvm-test-suite.

To reproduce, run `opt -passes=ipsccp` on the IR below.

    @g = internal global i32 256, align 4

    define void @test() {
    entry:
      %0 = load i32, ptr @g, align 4
      %div = sdiv i32 %0, undef
      ret void
    }