target-alpha: bug fix: 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.

Signed-off-by: Tristan Gingold <gingold@adacore.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6930 c046a42c-6fe2-441c-8c8c-71466251a162
