cmd/internal/obj/ppc64: eliminate ~700 conversions by tweaking signatures

This is mostly changing the opXXX helpers to take an int16 (matching Prog.As)
argument and return a uint32. The only bit that's not completely trivial is
passing -p.As to opirr to signal operating on a shifted constant, because AADD
+ ALAST overflows int16.

Change-Id: I69133800bbe41c38fa4a89bbbf49823043b3419c
Reviewed-on: https://go-review.googlesource.com/20182Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>