Lowering multiplyExact

[Andrew Haley]

Graal assumes that multiplies set the overflow flag, and relies on
this for multiplyExact.  AArch64 doesn't set the overlfow flag for
multiply, and as far as I can see neither does SPARC, so we have to do
some backward somersaults to get it to work.

Right now we do this:

  0x0000007fa8baec54: smull	x8, w1, w2
  0x0000007fa8baec58: add	w0, w8, #0x0
  0x0000007fa8baec5c: cmp	x8, w8, sxtw
  0x0000007fa8baec60: orr	x8, xzr, #0x80000000
  0x0000007fa8baec64: csel	w8, w8, wzr, ne
  0x0000007fa8baec68: cmp	w8, #0x1
  0x0000007fa8baec6c: b.vs	0x0000007fa8baec8c

which is

  x8 = i2l(w1) * i2l(w2)
  result = l2i(x8)
  flags = compare(x8, result)
  x8 = 0x80000000
  w8 = flags.ne ? w8 : 0
  flags = compare(w8, 1)
  ...
  if (flags.vs) goto overflow

But it could be simply

  x8 = i2l(w1) * i2l(w2)
  result = l2i(x8)
  flags = compare(x8, result)
  ...
  if (flags.ne) goto overflow

if we had a lowering for multiplyExact that didn't assume that
multiplies set the overflow flag.

It's really not an important thing, but it's kinda irksome.  :-)

Andrew.