[optimizing] Improve x86/x86_64 bound check code

Don't force a constant index into a register just to compare to the
array size.  Allow a constant, and compare the constant to the size.

Change-Id: I1c5732fbd42e63f7eac5c6219a19e9c431c22664
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>