x86/apic: KVM: Deduplicate APIC vector => register+bit math

x86/apic: KVM: Deduplicate APIC vector => register+bit math

Consolidate KVM's {REG,VEC}_POS() macros and lapic_vector_set_in_irr()'s
open coded equivalent logic in anticipation of the kernel gaining more
usage of vector => reg+bit lookups.

Use lapic_vector_set_in_irr()'s math as using divides for both the bit
number and register offset makes it easier to connect the dots, and for at
least one user, fixup_irqs(), "/ 32 * 0x10" generates ever so slightly
better code with gcc-14 (shaves a whole 3 bytes from the code stream):

((v) >> 5) << 4:
  c1 ef 05           shr    $0x5,%edi
  c1 e7 04           shl    $0x4,%edi
  81 c7 00 02 00 00  add    $0x200,%edi

(v) / 32 * 0x10:
  c1 ef 05           shr    $0x5,%edi
  83 c7 20           add    $0x20,%edi
  c1 e7 04           shl    $0x4,%edi

Keep KVM's tersely named macros as "wrappers" to avoid unnecessary churn
in KVM, and because the shorter names yield more readable code overall in
KVM.

The new macros type cast the vector parameter to "unsigned int". This is
required from better code generation for cases where an "int" is passed
to these macros in KVM code.

int v;

((v) >> 5) << 4:

  c1 f8 05    sar    $0x5,%eax
  c1 e0 04    shl    $0x4,%eax

((v) / 32 * 0x10):

  85 ff       test   %edi,%edi
  8d 47 1f    lea    0x1f(%rdi),%eax
  0f 49 c7    cmovns %edi,%eax
  c1 f8 05    sar    $0x5,%eax
  c1 e0 04    shl    $0x4,%eax

((unsigned int)(v) / 32 * 0x10):

  c1 f8 05    sar    $0x5,%eax
  c1 e0 04    shl    $0x4,%eax

(v) & (32 - 1):

  89 f8       mov    %edi,%eax
  83 e0 1f    and    $0x1f,%eax

(v) % 32

  89 fa       mov    %edi,%edx
  c1 fa 1f    sar    $0x1f,%edx
  c1 ea 1b    shr    $0x1b,%edx
  8d 04 17    lea    (%rdi,%rdx,1),%eax
  83 e0 1f    and    $0x1f,%eax
  29 d0       sub    %edx,%eax

(unsigned int)(v) % 32:

  89 f8       mov    %edi,%eax
  83 e0 1f    and    $0x1f,%eax

Overall kvm.ko text size is impacted if "unsigned int" is not used.

Bin      Orig     New (w/o unsigned int)  New (w/ unsigned int)

lapic.o  28580        28772                 28580
kvm.o    670810       671002                670810
kvm.ko   708079       708271                708079

No functional change intended.

[Neeraj: Type cast vec macro param to "unsigned int", provide data
         in commit log on "unsigned int" requirement]

Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Link: https://lore.kernel.org/r/20250709033242.267892-4-Neeraj.Upadhyay@amd.com
Signed-off-by: Sean Christopherson <seanjc@google.com>