AArch64: remove post-encoder method from FCMP (immediate) instructions.

The work done by the post-encoder (setting architecturally unused bits to 0 as
required) can be done by the existing operand that covers the "#0.0". This
removes at least one use of the discouraged PostEncoderMethod uses.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176261 91177308-0d34-0410-b5e6-96231b3b80d8