More fcopysign correctness and performance fix.

The previous codegen for the slow path (when values are in VFP / NEON
registers) was incorrect if the source is NaN.

The new codegen uses NEON vbsl instruction to copy the sign bit. e.g.
        vmov.i32        d1, #0x80000000
        vbsl    d1, d2, d0
If NEON is not available, it uses integer instructions to copy the sign bit.
rdar://9034702

llvm-svn: 126295