target-ppc: avoid nop to override next instruction

While searching PC, always store the pc of a new instruction.
Instructions that didn't generate tcg code (such as nop) prevented the
next one to be referenced.

Based on patch for target-alpha, r6930.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6931 c046a42c-6fe2-441c-8c8c-71466251a162
