Add FPExt option to CCValAssign::LocInfo. When generating calling-convention

Add FPExt option to CCValAssign::LocInfo. When generating calling-convention
promotion code, Tablegen will now select FPExt for floating point promotions
(previously it had returned AExt, which is not valid for floating point types).

Any out-of-tree targets that were relying on AExt being returned for FP
promotions will need to update their code check for FPExt instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199252 91177308-0d34-0410-b5e6-96231b3b80d8