The arguments get legalized to Reg or Mem, so we can allow constants
as well (including undef values). This change makes all instruction's
source arguments Ice::Operands.

BUG=swiftshader:24

Change-Id: I1659cdfdb1b8a12c4acc7c473211d8a67bfd5868
Reviewed-on: https://chromium-review.googlesource.com/418504Reviewed-by: Jim Stichnoth <stichnot@chromium.org>