cmd/compile/internal: improve handling of DS form offsets on ppc64x
In the ppc64 ISA DS form loads and stores are restricted to offset
fields that are a multiple of 4. This is currently handled with checks
in the rules that generate MOVDload, MOVWload, MOVDstore and
MOVDstorezero to prevent invalid instructions from getting to the
assembler.
An unhandled case was discovered which led to the search for a better
solution to this problem. Now, instead of checking the offset in the
rules, this will be detected when processing these Ops in
ssaGenValues in ppc64/ssa.go. If the offset is not valid, the address
of the symbol to be loaded or stored will be computed using the base
register + offset, and that value used in the new base register.
With the full address in the base register, the offset field can be
zero in the instruction.