[X86] Update MaxIndex test in x86-cmov-converter.ll to return the index and not use...

[X86] Update MaxIndex test in x86-cmov-converter.ll to return the index and not use the index to look up the array after the loop.

This represents a more realistic version of the code being tested.
The cmov converter doesn't look at the code after the loop so
it doesn't matter for what's being tested.

But as noted in this twitter thread https://twitter.com/trav_downs/status/1213311159413161987
gcc can turn the previous MaxIndex code into the MaxValue code. So
returning the index makes it a distinct case.