Change vixl's internal stack pointer constant

Change vixl's internal stack pointer constant

This was a little tricky. On ARM machines, register number 31 is the
stack pointer. However, in some encodings, register number 31 refers to
a virtual "zero" register (basically the /dev/zero of registers). To
make sure that you don't accidentally use the wrong one with the wrong
encoding, vixl uses register number 63 to identify the stack pointer
(does not exist in hardware) and converts it to 31 while assembling.

This means that vixl uses 33 different register codes, so I'm changing
PhysReg to reflect this. This means we can now only represent 31 SIMD
register on ARM. I think this is preferable over losing the stack
pointer / zero register distinction in vixl.

Reviewed By: @edwinsmith

Differential Revision: D1100167