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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146417 91177308-0d34-0410-b5e6-96231b3b80d8