Previously the constant-folding behavior for MO_S_Quot and MO_S_Rem
failed to narrow its arguments, meaning that a program like:

    %zx64(%quot(%lobits8(0x00e1::bits16), 3::bits8))

would be miscompiled. Specifically, this program should reduce as

                %lobits8(0x00e1::bits16)                 == -31
          %quot(%lobits8(0x00e1::bits16), 3::bits8)      == -10
    %zx64(%quot(%lobits8(0x00e1::bits16), 3::bits8))     == 246

However, with this bug the `%lobits8(0x00e1::bits16)` would instead
be treated as `+31`, resulting in the incorrect result of `75`.