Fix coding nit in x86amd64g_calculate_FXTRACT.

Fix coding nit in x86amd64g_calculate_FXTRACT.

The current code "return getExp ? posInf : posInf;" looks like a typo.
But when the argument is positive infinity then both the significand
and the exponent are positive infinity (there is a fxtract testcase that
checks that). So no need to check getExp. Just always return posInf
if arg == posInf, but add a comment explaining why.