Add preliminary support for vectors of integers by using the
`ValueLatticeElement::asConstantRange()` helper instead of a custom
implementation, and relxing various integer type checks.
This enables just the part that works automatically, e.g. icmps with a
constant vector operand aren't supported yet.
The change in ssa.copy handling is because asConstantRange() returns an
unknown LV for empty range, while SCCP's getConstantRange() returned a
full range. I've made the change to preserve the existing behavior.
6b76c1e64c