[XLA] Show cumulative cycle percent in xla_hlo_profile table.

Looks like:

  5624727 cycles (100.% 100?) ::       3865.8 usec  [...] TOTAL
  2121832 cycles (37.72% 38?) ::       1458.3 usec
  1932379 cycles (34.36% 72?) ::       1328.1 usec
   264366 cycles ( 4.70% 77?) ::        181.7 usec

The first line with the total is a little wird, but I figured it was
better to do it this way than to waste a precious character of
horizontal space.

I also considered rendering it as e.g. "?38%".  This is slightly more
expressive, but it gets hard to read pretty fast with two characters
smushed against both of the numbers.

I put the sigma at the end because I find it easier to read: With the
sigma at the beginning, its tips often blend in with the first number;
e.g. I find "?77" less readable than "77?".

Similarly I considered displaying more than two significant figures in
the percent, but since it's cumulative *anyway*, I didn't think these
were relevant.

This formatting is somewhat inconsistent with how we do the categories
tables:

     258 ( 6.68% ?87.81%)   non-fusion elementwise (12 ops)

I can change these to match if we want, but I sort of think of them as a
different case.  The categories tables have a lot more whitespace in
between entries (namely, one line per instruction in the category), so
noisiness is not nearly as significant a concern.

PiperOrigin-RevId: 207329731