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.

llvm-svn: 199252