Experimental support for aligned NEON spills.

ARM targets with NEON units have access to aligned vector loads and
stores that are potentially faster than unaligned operations.

Add support for spilling the callee-saved NEON registers to an aligned
stack area using 16-byte aligned NEON loads and store.

This feature is off by default, controlled by an -align-neon-spills
command line option.

llvm-svn: 147211