Fixing cast condition for removing casts from builtin FPClassification.

The function SemaBuiltinFPClassification removed superfluous float to double 
casts, this was changed to also remove float to float casts but this isn't 
valid in all cases, for example when doing an rvaluetolvalue cast. Added a
check to only remove if this was a conventional floating cast.

Added additional tests into SemaOpenCL/extensions to cover these cases

llvm-svn: 289650