decode: Make all update-form indexed loads and stores use RA_OR_ZERO

decode: Make all update-form indexed loads and stores use RA_OR_ZERO

Experimentation on POWER9 indicates that the invalid form of lbzux
with RA=0 uses just RB as the address, not R0 + RB.  Extrapolating
this to all update-form loads and stores with RA=0, change all the
update-form loads and stores to use RA_OR_ZERO rather than RA.

This then means that all decode ROM entries with insn_type = LDST
have input_reg_a = RA_OR_ZERO.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>