ABI: modify CreateCoercedLoad and CreateCoercedStore to not use load or store of

the original parameter or return type.

Since we do not accurately represent the data fields of a union, we should not
directly load or store a union type.

As an exmple, if we have i8,i8, i32, i32 as one field type and i32,i32 as
another field type, the first field type will be chosen to represent the union.
If we load with the union's type, the 3rd byte and the 4th byte will be skipped.

rdar://12723368

llvm-svn: 168820