ARM: intrinsic support for rbit.

We already have an ARMISD node. Create an intrinsic to map to it so we can
add support for the frontend __rbit() intrinsic.

rdar://9283021

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211057 91177308-0d34-0410-b5e6-96231b3b80d8
AArch64: Add backend intrinsic for rbit.

Define an intrinsic for the frontend to use and pattern match it to
the RBIT instruction.

rdar://9283021

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211058 91177308-0d34-0410-b5e6-96231b3b80d8