[SystemZ] Fix expansion of ISD::FPOW and ISD::FSINCOS

The ISD::FPOW and ISD::FSINCOS opcodes default to Legal, but there
is no legal instruction for those on SystemZ.  This could cause
LLVM internal errors.  Fixed by setting the operation action to
Expand for those opcodes.

Also added test cases for all other LLVM IR intrinsics that should
generate a library call.  (Those already work correctly since the
default operation action is fine.)

llvm-svn: 248180