Eliminate warning: variable set but not used

This patch silences a warning which occurs frequently in the
instruction decoder macros when an instruction does not use
all of its register fields, as in this example:

control.c: In function 's390_extract_secondary_asn':
control.c:889:13: warning: variable 'r2' set but not used
[-Wunused-but-set-variable]

The warning first appeared with the gcc 4.6 compiler.