x86/ioapic: Improve code generation for __io_apic_{read,write}()

x86/ioapic: Improve code generation for __io_apic_{read,write}()

The write into REGSEL prevents the optimiser from reusing the address
calculation, forcing it to be calcualted twice.

The calculation itself is quite expensive.  Pull it out into a local varaible.

Bloat-o-meter reports:
  add/remove: 0/0 grow/shrink: 0/26 up/down: 0/-1527 (-1527)

Also correct the register type, which is uint32_t, not int.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>