Bug 381274 powerpc too chatty even with --sigill-diagnostics=no.

Bug 381274 powerpc too chatty even with --sigill-diagnostics=no.

Even with valgrind --sigill-diagnostics=no (or -q) guest_ppc_toIR.c
will report various cases why it didn't handle an instruction. e.g.

  disInstr(ppc): found the Power 8 instruction 0x10000508 that can't be
  handled by Valgrind on this host.  This instruction requires a host
  that supports Power 8 instructions.

After which valgrind will generate a SIGILL. But in case the user uses
-q or --sigill-diagnostics=no they aren't interested in that diagnostics.
For example openssl will try some power 8 instructions while initializing
and catch the SIGILL if not supported without issue.

Guard those cases with if (sigill_diag) like the generic decode_failure.

git-svn-id: svn://svn.valgrind.org/vex/trunk@3398