[AArch64] Promote f16 SELECT_CC CC operands when op is legal.

[AArch64] Promote f16 SELECT_CC CC operands when op is legal.

SELECT_CC has the nasty property of having operands with unrelated
types. So if you do something like:

  f32 = select_cc f16, f16, f32, f32, cc

You'd only look for the action for <select_cc, f32>, but never f16.
If the types are all legal, but the op isn't (as for f16 on AArch64,
or for f128 on x86_64/AArch64?), then you get into trouble.
For f128, we have softenSetCCOperands to handle this case.

Similarly, for f16, we can directly promote the CC operands.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253344 91177308-0d34-0410-b5e6-96231b3b80d8