[Ada] Tidy up implementation of System.Fat_Gen.Valid and inline it again

[Pierre-Marie de Rodat]

Besides the improved commentary, the removal of superfluous subtypes and
other small tweaks throughout to make it more readable, this replaces the
use of T'Mantissa by the more appropriate T'Machine_Mantissa in the body
of System.Fat_Gen.Valid and also removes an unnecessary trick used to
distinguish zero from other denormalized numbers when T'Denorm is False.

This also inlines again the function, because it compiles to only a few
machine instructions for most architectures.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* libgnat/s-fatgen.ads (Valid): Add again pragma Inline.
	* libgnat/s-fatgen.adb (Valid): Improve commentary, tidy up left
	and right, and remove superfluous trick for denormalized numbers.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: patch.diff
Type: text/x-diff
Size: 10323 bytes
Desc: not available
URL: <https://gcc.gnu.org/pipermail/gcc-patches/attachments/20201214/04fcec94/attachment-0001.bin>