Improve overflow/underflow handling in regr_intercept().

Improve overflow/underflow handling in regr_intercept().

As with corr() and regr_r2(), improve regr_intercept()'s final
function to cope with overflow/underflow in the final calculation.
Here, instead of using sqrt(), we use frexp() and ldexp() to recover,
if an overflow or underflow is detected, so that the multiplication
and division steps operate on normalised mantissas, and cannot
overflow or underflow.

As with 6498287696d, and the previous commit improving regr_r2(), this
is arguably a bug fix, but given the lack of prior complaints, refrain
from back-patching.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Chengpeng Yan <chengpeng_yan@outlook.com>
Discussion: https://postgr.es/m/33E01656-BB3B-46E9-A41F-24A01A7C35F4@outlook.com