Accept alternative-base LDRS[BHW] as an alias of LDURS[BHW]

Accept alternative-base LDRS[BHW] as an alias of LDURS[BHW]

Many load instructions have two forms: LDR<x> that takes either:

- a register index or
- an unsigned scaled immediate offset

and LDUR<x> that takes:

- a signed unscaled immediate offset in the range [-256, 255]

The assembler usually maps out-of-range LDR<x> offsets to LDUR<x>
where possible.  GAS does this using matching OP_* codes; see
try_to_encode_as_unscaled_ldst in gas/config/tc-aarch64.c.

Some alternative-base Morello instructions also come in these
LDR/LDUR pairs, so we can use the same approach for them.

However, the alternative-base forms of LDRS[BHW] only support a
register index.  They do not have a register+unsigned scaled form.
There is therefore no OP_* pair linking alternative-base LDRS[BHW]
and LDURS[BHW] instructions.

This patch therefore treats immediate LDRS[BHW] as a straight alias
of LDURS[BHW].  Following existing practice, LDURS[BHW] is still the
preferred disassembly, so the patch uses F_P1 to force LDURS[BHW] to
be chosen ahead of LDRS[BHW].

Following the general preference for using immediate forms where
possible:

   ldrsb x0, [c0]

is treated as:

   ldursb x0, [c0, #0]

rather than:

   ldrsb x0, [c0, xzr]

Co-Authored-By: Stam Markianos-Wright <stam.markianos-wright@arm.com>