target-alpha: add instruction name in comments for hw_ld opcode.

Make code slightly easier to read.
Also unused hw_ld opcodes now generate an invalid opc fault.

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@6928 c046a42c-6fe2-441c-8c8c-71466251a162
