Implement 'e' and 'f' modifiers for Neon inline asm. <rdar://problem/10551006>

These modifiers simply select either the low or high D subregister of a Neon
Q register.  I've also removed the unimplemented 'p' modifier, which turns out
to be a bit different than the comment here suggests and as far as I can tell
was only intended for internal use in Apple's version of gcc.

llvm-svn: 146417