Use ArrayRef(WithPadding) in constraint code

Use ArrayRef(WithPadding) in constraint code

This change is only refactoring, except for adding padding to two
buffers used for flexible constraints. This avoid potential illegal
memory access one element beyond the buffer with certain SIMD
implementations. Note the this memory was not used nor modified.

Change-Id: I385b3007a27888d15741e737ccfcf3e3a4369d1e