rs6000.md/altivec.md problem in setting of vector registers

[David Edelsohn]

>>>>> Dorit Naishlos writes:

Dorit> I focused on understanding what in the machine description explains the
Dorit> different ways Reload handles the same pattern ('set subreg') on the two
Dorit> platforms (i386/powerpc).

	Altivec and SSE are integrated in their respective architectures
in different ways, so GCC of one is not alway appropriate for the other.
The vec_set and vec_extract patterns provide explicit control over setting
vector elements, so that probably is the best way to achieve the optimal
behavior.

David