[AArch64 NEON] Lower SELECT_CC with vector operand.

When the scalar compare is between floating point and operands are
vector, we custom lower SELECT_CC to use NEON SIMD compare for
generating less instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200365 91177308-0d34-0410-b5e6-96231b3b80d8
Use a raw_stream to implement the mangler.

This is a bit more convenient for some callers, but more importantly, it is
easier to implement correctly. Doing this removes the patching of already
printed data that was used for fastcall, fixing a crash with private fastcall
symbols.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200367 91177308-0d34-0410-b5e6-96231b3b80d8