x86: improve array_index_mask_nospec() code generation

Don't force the inputs to be 'unsigned long', when the comparison can
easily be done in 32-bit if that's more appropriate.

Note that while we can look at the inputs to choose an appropriate size
for the compare instruction, the output is fixed at 'unsigned long'.
That's not technically optimal either, since a 32-bit 'sbbl' would often
be sufficient.

But for the outgoing mask we don't know how the mask ends up being used
(ie we have uses that have an incoming 32-bit array index, but end up
using the mask for other things).  That said, it only costs the extra
REX prefix to always generate the 64-bit mask.

[ A 'sbbl' also always technically generates a 64-bit mask, but with the
  upper 32 bits clear: that's fine for when the incoming index that will
  be masked is already 32-bit, but not if you use the mask to mask a
  pointer afterwards, like the file table lookup does ]

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>